Well, I think on a tourney, more is at stake than when you play say, just for fun, and that applies to different activities. It's not just about say, getting the prize, but more about a self ego boost, when you with a tourney you feel great about yourself, you feel like you've achieved something great and are better than the rest of the participants. Focusing on winning will end up making you feel nervous, you put pressure on yourself, causing you to get nervous, and unconsiously mess up.
When I used to play sports I used to try and focus on having a good time rather than focusing on the win. I would perform better and enjoy the sport even more.
I think I feel more under pressure when I play with friends rather than when I play on a tourney, my friends know me really well, and they know what ticks me or how to **** me off, and if I lose they'll give me sh*t about it, whereas on a tourney, you go after a prize, mostly playing against people you don't know, and with an aduience that doesn't really know you, so it's less pressure, for me anyways...