That is a good question but you might want to word it to be a little clearer that what you mean is either the cause of them trying to be beautiful or why we require people to be beautiful. Try these:
What causes people to spend time and money on their appearance?
What causes women to feel they need to look beautiful?
What causes men to feel they need to look attractive?
What makes us be more attracted to beautiful people?
Why can beauty make someone famous?