Why do people try to think positively?

Positive Thinking – I’ve never been a fan of it myself and often find when thinking everything’s going to go wrong it will still work out (or at least be bearable). So there seems to be (to me) no pro’s in it.. and isn’t unhappiness just a part of the human condition? I don’t understand why so many people subscribe to this, especially people who try to change the way they think to match this programming.