Some web economics. Advertisers get paid for a click, and also for a page view. Many websites, make attempts to refresh the page as often as possible. Pandora refreshes the advertisement even when you do something as trivial as changing the volume, and surely with a thumbs up, thumbs down or skip song. Other websites, like news websites have those little "next page" buttons. With all their heavy scripting they could certainly fit a few more ascii characters on the page but they want you to click so they can show another add. For the record I have nothing against this practice, I am just commenting on it.