P42: Cheerfulness cannot be excessive, but is always good; Melancholy, on the other hand, is always evil.

Dem.: Cheerfulness (see its Def. in IIIP11 S) is a Joy which, insofar as it is related to the Body, consists in this, that all parts of the Body are equally affected. I.e. (by IIIP11), the Body’s power of acting is increased or aided, so that all of its parts maintain the same proportion of motion and rest to one another. And so (by P39), Cheerfulness is always good, and cannot be excessive.

But Melancholy (see its Def., also in IIIP11 S) is a Sadness, which, insofar as it is related to the Body, consists in this, that the Body’s power of acting is absolutely diminished or restrained. And so (by P38) it is always evil, q.e.d.