IMO it is largely a depression issue.

Depression makes our brains tend to ignore anything even remotely positive and focus heavily on every little (or big) thing negative.

So if you can take it upon yourself to think about, remember, and appreciate those good things, even if small, it is very healthy for your brain and maybe even help toward recovery and dealing with the big stuff better.

Also, in general, life is a series of good feelings and bad feelings, and you wouldn't want to ignore the good feelings just because there are bad feelings sometimes.

With depression though it sways the balance of what we see as good/bad feelings and that is what sucks so much with this illness.