though it seens wrong to quote wikipedia I liked the various theories on this page discussing why sadness or depression may have evolved in the first place (the page does at least include links to peer reviewed articles and experiments)
http://en.m.wikipedia.org/wiki/Evolution...depression
just remember sadness can serve a distinct adavantage over other states of mind and happyness would be innapropriate if expressed all the time. your also at an age that can generally get caught up in melancholia and again that might be an evolutionary thing. also high dependence on surrounding, age, experience. at fifty you might be the most mellow person ive ever met in my life lol. I know a few things that seemed very important a few years back that as time goes on just drop of the radar.
http://en.m.wikipedia.org/wiki/Evolution...depression
just remember sadness can serve a distinct adavantage over other states of mind and happyness would be innapropriate if expressed all the time. your also at an age that can generally get caught up in melancholia and again that might be an evolutionary thing. also high dependence on surrounding, age, experience. at fifty you might be the most mellow person ive ever met in my life lol. I know a few things that seemed very important a few years back that as time goes on just drop of the radar.