(condensed from Ein Ayah, Berachot 9:32)

Gemara: Rav Chisda said: All dreams, and do not fast.

Ein Ayah: One fasts due to a dream only when his spirit is depressed by it even after he awakes. Rav Chisda was making the following point in regard to one’s reaction to dreams.

All the natural things that happen to a person’s spirit are for his betterment, as everything Hashem does is for the good. Since dreams are a permanent, natural human phenomenon, certainly dreams are good for a person’s body and spirit. However, natural phenomena are good only when they stay within their natural boundaries, for when they extend beyond, they can be damaging. Specifically, dreams are supposed to impact on a person as he sleeps. When they are so powerful as to have such an effect that they depress a person to the point that he wants to fast, it has certainly gone too far. Such an exaggerated feeling is never good and is a sign that something is wrong with the person’s emotional state.