Source: Malbim's commentary on Megillat Esther, translated by Jonathan Taub
Before recounting the troubles that befell the Jewish people, we are told that "the cure preceded the wound". We are taught that the Almighty only strikes us in order to heal us. This is the meaning of the verse: "All the diseases which I brought on Mitzrayim I will not place upon you, because I am Hashem Who Heals You." [Shemos 15:26]
When the messengers sent by Hasach gave Esther's instructions to Mordechai, Mordechai told them to reply to Esther: "Do not think to yourself that you can escape in the king's house more than all the Jews." [Megillat Esther 4:13]
Why did Mordechai say "Do not think to yourself" when "Do not think" would have sufficed?
Mordechai informed Esther of several fundamental principles:
1) Any time something unusual and extraordinary occurs, we must realize that the Almighty has brought this about as a means to achieving some important providential end. Esther's remarkable elevation to the position of queen was clearly Divinely determined for some significant purpose. With the revelation of the decree to annihilate the Jewish people, it was now obvious that Esther was intended to help save the Jews.
2) For any tragedy that befalls the Jews, there is a specific time and day ready for their deliverance by certain prepared means. If these means are not actualized, then the Almighty will prepare other means to save them.
3) Anything that does not exist for itself, but to help something else, has its entire existence dependent on what it was destined to help. For example, if a gardener wants to plant certain flowers in the winter, he will construct a special greenhouse with a heating system to help these flowers grow. If he changes his mind and wants the flowers to flourish in the summer, when there is an abundance of natural light and heat from the sun, he will dismantle the greenhouse, for it is no longer required.
Mordechai was telling Esther that her position in the royal house was not for her own benefit, but for the sake of the Jews. If Esther decided not to help rescue them, the Almighty undoubtedly had other means and methods of bringing about their redemption, regardless of how improbable they might seem.
Since Esther was granted her position as queen only in order to help the Jews, and since the date for their salvation had already been determined, if she did not act immediately she might miss the appointed time and the Almighty would use other agents to affect their deliverance.
This was Mordechai's warning to Esther. Do not think that your elevation to royalty is to yourself, for your own benefit, so that you can escape in the king's house, rather than for saving the Jewish nation. The opposite is true: it is only to enable you to rescue the Jews.