We feel happy when our expectations are met. And, we feel disappointed when they aren't. By working hard for something, we shift our expectations up- by working hard at a job, we expect better results, or whatever. The crux of my argument is that the harder we work for something, the more we expect. and therefore, the more disappointed we are when we don't get it. Also, the harder we work for something, the less excited we are when we do get it, simply because our expectations are a lot higher. If one doesn't work hard, one doesn't expect very much- and is, therefore, more likely to be pleased.
I'm in college right now- I know students who study for days before tests, and then there's a few like me who just sort of show up, and do reasonably well anyway. Those who study a lot do feel happy when they get good marks, but they feel absolutely devastated when they don't perform as well as they hoped they would. As for the lazy lot, we don't really care if we get worse marks because we know we don't deserve them anyway. And, if we do perform well, we're absolutely delighted- "you know, i got 80% without studying at all".
So, if one works hard, then one can either feel devastated, or be mildly satisfied... and if one doesn't work too much, then one either doesn't care, or feels awesome. If your aim in life is marks(/success/money), by all means go ahead and study(/work) but if your aim is to be happy and satisfied with yourself, laziness is the way to go!
PS. This doesn't really work in the long run. For true happiness, learn from Fight Club!