People are not saying they click on their own ads, they are saying about clicking on other peoples ads if they find something interesting. I know you shouldn't click on your own ads.
i guess every clicks comes from search engines as use comes to search some infos.like they search form google the infos they wanted and come to your sites and they see more good infos about they are searching on google ads god they just can't stop clicking and we just can't stopping jumping and laughing that we got clicks
I remember before I learned about Adsense, I would click on them all the time, just because it was something that interested me. I can see why MySpace-related pages make a lot of money, it is a bunch of kids/computer-illiterate adults surfing the net, so they don't even realize when they click an ad.