> Generally the people who were most concerned about others well-being were gifted with a light and pleasant demeanour

Kinda like the light and pleasant demeanor of Robin Williams? This is crazy ovservation bias, you have no idea of the actual, lived experiences of these people. Honestly, my experience is that people who really suffer with depression are consistently some of the nicest, most thoughtful people. They understand how shitty it is to be down and do as much as they are capable of to try and stop the spread and help themselves, and being appreciative and positive are, as you point out, pretty excellent at making one feel better. People who are constant downers and negative, in my experience, are more self-obsessed and trying to get something.