What makes people think they are better at something than they are?

I’m watching “So You Think You Can Dance.” Some of these people are at a very low skill level, yet they still showed up to compete with the best in the country. What in the world makes people think they are good enough to get on a television show like this? (American Idol also falls in this category).
Does it come from an outside influence? from within?

This applies to everything btw, not just singing and dancing