Why do people make winning the only part of their lives?

Every time I get together with my cousins we always play some kind of a game. Whether it's inside playing Monopoly, or going outside and playing some sport. It's a perfectly harmless game until somebody decides that things aren't going their way and they quit. That's usually how it goes with one of my cousins, and frankly I think it is the most annoying thing ever. Every little time the rules are changed slightly he pouts and leaves the game because he knows he can't win at that point. So if people can be taught that winning isn't everything and that people are allowed to have fun, then life will be a whole lot more fun.