Why is it the ugliest Advertisments that seem to show up all over the web?

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It seems like whenever I see a real ugly advertisement, I end up seeing it on multiple websites all over the web all of a sudden. This weeks ugly add is the girl lifting her shirt to show her fat belly and then her after belly. They had to pick the worst belly ever b/c it's wrinkly, and saggy, and gross! Even overweight ppl I know do not have bellies like that. Suddenly I'm seeing this same Ad on at least 4-5 different sites this past week, including Mylot. Why do these things always spring up all over at once? And why do they think that ugly picture will pull me in? If they wanted to sell me diet products, they should just show me a real thin attractive person... not a side view zoomed in of someones ugly "before" belly. Don't you wish that you could block certain ads from your computer?