In order to receive Hashem's blessings, a person needs to make a vessel (a keili) capable of receiving them. The way to do this is by observing the mitzvot, and prayer. In that way, the person becomes a suitable keili - a vessel that can receive and contain blessings.
On a larger scale, there is no greater keili than achdus - unity.
Someone who receives a bracha (blessing), or a nes (miracle) but doesn't live up to their expectations on a spiritual level, can cause the keili to crack, and the blessings to leak out - they cannot contain the blessing because their keili is broken. To fix the problem, they need to repair the keili - go back and do teshuva.
Regarding financial blessings: A Jew's income is determined on Rosh Hashanah. If that's the case, then why should we work? After all, our income for the year has already been decided....
"It is the way of Hashem that His blessing must flow down in a natural way. For whatever reason, it is His requirement that, even when nature is suspended, the suspension is through nature and in a way which is apparently natural. In order to receive the blessing, man must make a keli (vessel) to contain that brochah. The vessel must be part of nature so that the brochah devolves through apparently natural means. The keli for parnossah is work. This is the reason, and the sole reason, a Jew is required to work." [from "The Ladder Up" by Robert Kremnizer]
Making a 'Keli'
The Medrash explains that one should not say, “I will eat, drink and enjoy, and in Shamayim they will have mercy,” for Hashem sends his bracha through a person’s work, and without doing, one will not receive the bracha. This can be learned from Yitzchak who planted his field so that Hashem’s bracha would be able to settle there.
The talmidim of Rebbi Shimon Bar Yochai asked him: “When the Yidden were in the midbar, why didn’t Hashem make enough מן come down once a year to last for a full year?” Rebbi Shimon explained this with a moshol: A king had an only son whom he supported by giving him his needs once a year. However, the king was unsatisfied with this arrangement, for the prince would only visit once a year to receive his provisions. Therefore, the king decided to give him his daily needs on a day to day basis, and now the prince would have to visit his father every day. So too, a Yid living in the midbar who had a number of children would worry and say, “Perhaps the מן will stop coming tomorrow and we will all die of hunger.” He had no choice but to put his full trust in Hashem, that He would provide him with all his needs. [יומא עו ע״א]
Chazal say: The One who has created each day, also created its parnasa. Rebbi Elazar Hamoda’i would say that one who has food for today, yet worries what he will eat tomorrow, is lacking in emunah. The Maharsha explains that this is only applicable to great ‘chassidim’, whereas all others are allowed to be concerned and daven to Hashem. [מכילתא בשלח, סוטה מח ע״ב]
The Alter Rebbe writes: Bracha and success come from above and the only thing necessary to be done is to make a ‘keili’ (vessel) for this bracha. One who is very involved in his parnassa is comparative to a person who sews clothing for himself that are too long, causing himself to trip and fall.
The Tzemach Tzeddek writes that one who is overly involved in business is similar to one who sows many wallets so that he can have a lot of money... So too without the bracha of Hashem, the business is an empty wallet. [לקוטי תורה תצא לז, ב, דרך מצותך מצות תגלחת מצורה]
Only a 'Keili'…
On one of his travels, the Baal Shem Tov went up to a house, knocked on the window and then continued on his way. Hearing the knock, the one living there rushed out of his house and caught up to the Baal Shem Tov, asking him what he wants. The Baal Shem Tov told him that he needs a certain sum of money and the man fulfilled his request.
The talmidim of the Baal Shem Tov asked him, “If there was a need to knock on the man’s window because something was needed from him, then why did you leave right away without waiting for the owner to come out and hear your request?”
The Baal Shem Tov explained that Hashem is the One Who fulfills one’s request, but He wants that man should also do some action on his part. Therefore it was enough to do something small like knock on a window, and once he had done his part, he had no reason to stay and therefore had continued on his way.
The Rebbe explains that it is up to the person to decide how much of an effort he will have to make, whether he will learn all day and do only ‘something’, or suffice with a little learning in the morning and evening, and work the rest of the day… [התוועדויות תשמ״ז ח״א ע׳ 290 , לקו״ש ח״ה ע׳ 34]
The mashpia Reb Mendel Futerfas related: At one point, the Yeshiva Tomchei Temimim in Russia was in such dire straits that even Reb Chatche Feigin, normally very straightforward and organized, had to avoid those who had loaned funds to the yeshiva. The situation was so severe that Reb Chatche once jumped out the window of the shul in order not to meet the shamash who had lent the Yeshiva some money. During this time, when we would ask him, “What’s going to be?” he would reply, “Why should you worry? It is clear, that in the end everything will be as Hashem has ordained. It is not our issue; we must do what is required of us, and Hashem will do as he desires…” [ר׳ מענדל ע׳ 262]
During the First World War, a chossid asked the Rebbe Rashab if he should sell the forest he owns, for the German army was approaching, and the forest would likely be lost. The Rebbe Rashab advised him not to and explained: “The Mezritcher Maggid said that if one has a functional ‘keili’ for Hashem’s bracha, he should not break it. Only if from Shamayim they cause it to break, then there is no other choice but to look elsewhere…” [רשימת דברים ח״א ע׳ קס״ט]
The Rebbe explains that also with regard to spreading Yiddishkeit, the person’s doing is merely a ‘keili’ and the success comes from Hashem. However, in this case a person must do with much effort and labor, and then they can merit the supernatural hatzlacha. [התוועדויות תשמ״ב ח״ב ע׳ 56]
There was once a man who was both a skilled craftsman and an accountant, but was unsuccessful in earning money. The Rebbe of Kotzk once called him over and asked, “Do you understand the possuk, ‘ לא לחכמים לחם ’ (simply meaning that though one is wise, he may not have bread)? The man was quiet, and the Rebbe explained “Hashem is telling a person, ‘If you think you are a chochom, then go look for your parnasa yourself…’” [סיפורי חסידים זוין ]לה״ק[ מועדים ע׳ 15]
A Proper 'Keili'
Chazal say that although a person should have a trade, he must daven to Hashem, for any trade can either succeed or not, and it is dependent on a person’s zechusim (merits).
Rebbi Shimon ben Elazar said, “Have you ever seen animals working for a living: perhaps a deer working in an orchard, a lion as a porter or a fox as a storekeeper? Despite their lack of work they still have provisions, while I have to labor for mine! It is only because I myself have caused this, through my aveirois (sins).” [קידושין פ״ב ע״א]
The Torah says that a person must remember that it is Hashem Who brings a person any success, and one should not attribute it to his own doing. Some list this as one of the 613 mitzvos. The Rebbe gave the example of a businessman who before davening in the morning, hurries to call his associate and strike a deal, lest someone else precede him. He does this because he thinks that he is the one who brings the parnasa. If he would truly believe that everything is from Hashem, he would work only because Hashem has commanded and do so only in the manner in which he was commanded, for it is unthinkable that following the directives of Hashem will bring him a loss. Behavior contrary to this is a subtle form of Avoda Zara! [עקב ח, יז, סמ״ג מל״ת סד, התוועדויות תשד״מ ח״ג ע׳ 212]
A simple Yid once traveled to the tzaddik Reb Mordechai of Chernobyl for Shabbos. When he went to get a bracha before departing, Reb Mordechai asked him about his daily routine, and the Yid told how he rises early to buy merchandise from the farmers and returns home to daven when he finishes. The Rebbe condemned such behavior, but the Yid excused himself saying that if he davens first, he will be unable to buy the merchandise. Reb Mordechai then told him the following story:
“A yungerman who was supported by his father-in-law, was forced to find additional means of support when his family grew. He left home for three years and earned money as a melamed, saving every coin he received. When he had collected enough to start a business, he decided to return home. On Erev Shabbos, he reached a small village near his city and realized that he did not have enough time to reach home before Shabbos, so he decided to stay at a motel. He was afraid to leave the money in his bag, lest someone steal it, and he did not know if he could trust the owner with it, but having no other choice, he gave it to his host to hold for him. Throughout the entire Shabbos, the yungerman worried about his money, and immediately after Havdala requested it back. Upon receiving his wallet, he counted all the gold coins and was happy to find that nothing was missing. He then continued to shake the coins and look through them. “What are you looking for?” the owner asked, “Is something missing?” The guest told him that he wanted to make sure that his one copper coin is there as well…”
Reb Mordechai concluded, “Look at this man’s silliness. After seeing that all his golden coins were returned to him, he still suspects his host of perhaps stealing one copper coin… And you are doing the same. Every night, you entrust Hashem with your neshama, and when you wake up in the morning, he returns the gold you have given him. How is it that you do not trust that he will give you your parnasa if you will wait until after davening…?” [אדמו״רי צ׳רנוביל ר״ה]
Returning home from cheder, on his way to his father’s room, the Mitteler Rebbe saw Reb Shmuel Munkes among other chassidim and ran towards them. Listening to their conversations, he heard Reb Shmuel ask two wealthy chassidim why they looked so downcast, to which they responded that they were experiencing some hardships in parnasa. The young boy was surprised at the question, and claimed that this type of worry is clearly described in Tehillim. He quoted the possuk “ עצביהם כסף וזהב, מעשה ידי אדם ”, (simply meaning that the Avoda Zara of goyim are made of gold and silver, fashioned by hand) and interpreted it to mean that people are sad (עצב) because they think their gold and silver is dependent on a person’s actions. The Mitteler Rebbe continued, “They are so foolish that they think the quicker they hurry to bring merchandise from the fairs and do more business, the more money they will accumulate.
The businessmen’s blindness causes (as the possuk continues) that פה להם ולא ידברו , though they have mouths and repeat chassidus, it does not change them; עינים להם ולא יראו , they have eyes, but do not recognize Hashgocho Protis (Divine Providence); they have ears but only hear the chitzoniyus (superficialities), and therefore have no ‘sense of smell’.” The Mitteler Rebbe concluded, “And so they become ‘avoda zarah’…” [לקוטי דיבורים ח״א ע׳ 340]
The Frierdiker Rebbe writes: When the Yeshivah Tomchei Temimim was established, my father, the Rebbe Rashab instructed that when raising money, they should not overemphasize the greatness of the Yeshivah in order to increase the contributions, “We must only do as Hashem commanded and make a Keili by notifying Anash of the Yeshivah and its nature.” [אג״ק ריי״צ ח״א ע׳ רכו]
תנחומא ויצא, תדב״א יד, תוספתא ברכות פ״וSource: L'Maan Yishmeu