Having tons of ads makes your site look messy. Visitors don't like seeing a cluttered mess of ads when they come to your site. Especially if the ads are annoying (pop up ads, for example) or block the content the visitors are trying to read. Also, flashy ads and video ads are also not very good as they make your web pages bigger and slow down your site load time. And if your site is hard to navigate because of all the ads, people will quickly lose interest. So, it's best to keep things balanced and not overdo it with ads on your site.