Life does not always go smoothly. Just as a person’s [mood or circumstances] can go up, there is also the possibility that they can go down. One need not be shaken and forlorn when he does experience a downturn in mood or status, especially when it only involves material matters, and particularly when he knows that he has no control over these things. This downturn should serve as impetus to strengthen and deepen his trust and belief in G‑d — so much so that an outside observer would see that he is the stronger for it and does not get shaken by undesirable outcomes in his life, especially since it is only for a short period of time.

Igros Kodesh of the Rebbe, vol. 8, p. 129