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.

The Impact of the Reaction to a Dream
(condensed from Ein Ayah, Berachot 9:34)

Gemara: Rav Chisda said: It is sufficient for a bad dream [Rashi- to nullify it] that one is sad; it is sufficient for a good dream [Rashi- to nullify it] that one is happy. Rav Yosef said: Even for me, it is sufficient for a good dream that happiness nullifies it.

Ein Ayah: Natural phenomena leave their mark work on the spirit but only while the spirit stays in its natural state. However, when an external stimulus alters the person’s state, then the natural phenomena will cease to have their normal impact. The weaker the natural phenomenon, the easier it is to remove its impact. A dream is a natural stimulus but a weak one, which is easily nullified by small external stimuli.
A normal person is affected by the things that he sees during his waking hours, so that those sights take away the impact of his dream. Rav Yosef was blind, though, and thus the relatively strong stimulus of sight was missing. Despite that fact, the reaction to the dream alone was enough to upset the impact of the dream because the dream’s power is weak. Because he was blind, there is reason to believe that Rav Yosef would be in a sullen state in which his reaction to a bad dream would have more of an impact than his reaction to a happy one. Therefore, he stressed that even his reaction to a happy dream was enough to undo the natural impact of the dream.