They might not ever admit to it, but even though you might look great when you compete, you feel like CRAP!!! I remember I was so depressed when I was dieting. Other people tend to get grouchy or really moody when they are dieting. It's not healthy to continue in a depleted state for a long period of time, even when you are natural. On the otherhand, it's a high like no other. It's one of the greatest accomplishments you can take your body through (running a marathon or doing an triathlon is probably greatest in my book). You meet some really cool people. Everyone wants to have a good time after the show (except those who are pissy about not placing higher).
One word of advice if you've never competed before... Be sure you are in the right frame of mind, i.e., make sure you are comfortable with who you are prior to competing, because bodybuilding is all about critique. Realize that everyone (even the best in the world) has natural flaws. And don't compare yourself to every other competitor on the stage.