What is with all the teen questions about the size of their penises?

Good question: It probably has to do more with culture and mis-information than anything else. Small penises enjoy sex, large penises enjoy sex, or is it the partner who is complaining? Varies from individual to individual. It is usually not size, but technique that causes satisfaction. Society is a bit extroverted on sex and too much narcissism abounds. Take what you have, be responsible and enjoy sex!

Expectations: Movies and quips about "how big somebody is" and watching the women there get excited about it adds to the expectation that "bigger is better." thus men think that being larger is important, and they keep talking about it. But like most male myths, women have a wide range of expectations and likes, and often being very large can be more curse than anything. Plus, you can't change sizes.