I don't think it's necessarily anything I do, but I tend to be happy regardless of what's happening, simply because the alternative sucks. It's not really a forced happiness, but it is chosen, and after a while it becomes completely natural to just be happy for no goddamn reason other than that being happy makes me happy.
You know, you're completely right. Why the hell do I choose to be upset and unhappy? It sucks. I'm going to focus my efforts to just be happy. Because you're right, it is a choice. Thank you.
Just a warning about this kind of mindset: I often end up in a sort of contentment with my lifestyle, regardless of how unhealthy it is. A certain amount of discontent is good for motivation and building up that motivation for an unpleasant activity (such as studying) is difficult when you're happy regardless of the fruit of your labor.
"Most folks are about as happy as they make up their minds to be." - Abraham Lincoln