It's all about passing down excellent genes. Even if you are not actively having sex to reproduce, you will benefit from the positive reinforcement and chemical release that sex provides. Of course, if your personality is completely crap, it won't matter, but, most people don't have the worst personality ever. If you come from a problematic family (as the majority of us do), there isn't much in this world for you. Some people can be happy alone, while others are simply not built for it.
