Rabbi Nota of Chelm had a chassid who was very well-to-do, who said to him "I am very wealthy and I lack for nothing. But recently there is a little voice within me, that tells me that all is not well. It is as if I have a premonition that my fortunes are about to take a turn. It is one thing if a person is born into a life of poverty, and accepts poverty as a way of life. Not so with me. If I lose my fortune, the change will be disastrous, and I doubt that I will be able to adjust to it."
"What kind of changes have you made in your home recently?" Rabbi Nota asked.
Assuming that the Rabbi was inquiring about laxity of Torah observance, the chassid said "G-d forbid, Rabbi. Everything is as it was. Shabbos is totally Shabbos, my kosher standards are as rigid as ever, and I faithfully study the Torah daily."
"That's not what I am after" the Rabbi said. "What physical changes have you recently made in your household?"
The chassid thought for a moment, then said "Yes, I did make a change, but it is hardly significant. I had a set of expensive crystal glassware, but I would get upset when a crystal goblet fell or was chipped. I therefore set it aside, and bought silver goblets which are more resistant to damage."
"There you have your answer" Rabbi Nota said. "Every person is destined to experience a small amount of adversity. You were fulfilling your quota of unpleasantness when a piece of crystal was damaged. When you eliminated that source of unpleasantness, you invited adversity from other sources. Put away the silver goblets and use the crystal again. You will then have sufficient aggravation from the crystal being chipped so that you will not need any other."
And so we have an explanation for the custom in Jewish homes that when a glass or dish breaks, we exclaim "Mazel tov!" If we were destined to experience some loss, we satisfied this decree by the loss of the glass or dish, and now we could go on to be happy.
Source: Rabbi Abraham J. Twerski MD