Three, count 'em, three simple rules that explain everything about human nature. Rely on these rules, and you can quickly and clearly figure out just why the hell people do the things they do.
People are strange.
People are stupid. This somewhat harsher rule covers those things that are very possibly explicable, yet are mindbogglingly inane, or inexplicable things that just rub you the wrong way. People are messy. The domain of this rule is not only voluntary actions, but also many biological actions as well. It covers those acts that aren't necessarily strange or stupid, but are just a bitch to clean up afterwards.