I've always fancied myself a decent game designer. Whether it's conjuring up a Greek mythology-based tabletop RPG for a high-school English class project, or babysitting my cousins using some novel, improvised games involving Nerf guns, I tend to think about what makes things fun for people. A great site that features plenty of articles on the why and how of what makes some games great and others bad is Sirlin.net (the blog is also quite interesting).
My favorite articles on the site are the ones devoted to "Playing to Win." I suppose everyone at times has been accused of "gamey," "unfair," or "cheap" behavior - but a competitive game demands this kind of play in order for you to win. After all, it's not like you're cheating - you're just playing to win. It's a neat reflection on human motives that even stretches to stuff like Survivor.