My old boss (I worked at a neighborhood music shop) used to say, "I've got my guitar, I've got my truck, and I've got sunshine. It doesn't get any better." Of course when you're 17, you're thinking "What about beer and chicks?" Took me a couple years to finally figure out what he was talking about.
People who are very, very happy are because THEY WANT TO BE. So here goes; my job is boring, I'm sick of school, just broke up with my girlfriend, I've got the flu, and I just recovered from loosing about a year of my youth (long story). And guess what, I'm happy. Not because life just seems to be going my way, it's because I choose to be. I can sit around all day complaining or being depressed, but how is that going to make me happy? How is that going to fix aspects of my life that suck? I've tried it before and it doesn't help.
Here's a little trick I use that keeps you possitive (and helps in every aspect of life); have a sence of humor about bad things and over-appreciate good things. Here's an example:
I'm sitting at work listening to my radio and Limp Bizkit comes on. Instead of thinking, "God damn it! They play this crappy song all the time. What the hell is wrong with them. blah blah blah", I can just listen to it and try to find a good aspect of it. If I can't, I love to listen to it and laugh at just how bad it is. After that, they might put on some Hendrix, so I can turn up the radio and appreciate that they are playing stuff I like. Just simple stuff like that helps a ton.
It's easy to keep writing about this sort of stuff, so I just stop here. Sorry to kind of go off on a tangent there...