It's interesting that YouTube shows ads only for videos created by partners (media companies, popular video creators), a small portion from the total number of videos uploaded daily.
Here's a funny video targeted to the potential advertisers on YouTube that explains how you can advertise on YouTube and why you should:
The YouTube community watches hundreds of millions of videos every day, more than any other TV network, cable network or social network combined. YouTube isn't just about media impressions, it's about the choices people make to be entertained, educated, inspired. It's about tiger vs bear. It's about sharing.
Think of YouTube as the world's largest magazine rack. If it's of interest to somebody, it makes its way here.
YouTube thinks there's a misconception that the site is just about user-generated content and mentions that advertisers can place their messages next to high-quality content that won't diminish the value of their brands.