I can only give you my personal experience and then you work out what is happening.
When I click on anything, I don't actually conciously watch the item unless it really catches my attention. With youtube videos, more often than not, I get a really long advert in Greek (which I don't understand) and I'm too lazy to stop it until the timer runs out. So, I don't often see the real videos that people are posting. I have plugins that stop the in-video ad banners, but I haven't found anything yet which kills the starting video-advertisement. Other than in fanslave, I would never watch youtube anyway - I've too many bad experiences with Google in general.