I'm in the Robin Williams category of depressives. I know how desperate and tiring it is fighting off those causeless bouts, and so I try to make life as enriching as I can for people around me.
It's my way of compensating for every relentless, hammering, baseless urge to die that I've survived alone over the years. It took a long time for me to get that people don't typically understand that it's more like hiding that you have cancer than not being able to cheer up. You can hide the symptoms of both, you don't ask for either of them, and they might both kill you, but somehow you're responsible when you get depression but not cancer.