For me it's mainly solitude, ugliness because of losing my hair and lack of drive resulting from that, isolation etc. Depression resulted from that, it's not a cause per se, in the exceptional 2 months when i had a gf for instancei felt happy and motivated. I'm only speaking for myself here, but my "depression" if there is any depression is mainly a situational depression, caused by real life factors, by an objectively shitty life of isolation and mediocrity and lack of bonding, not by "chemical imbalances" those are the results, not the cause.