This comment assumes people can choose what they think about when most of the time there doesn't seem to be an off-switch to a certain topic or emotion. I've found that it's easier to re-apply one's thought process on something mentally demanding and important (so the mind will find it urgent enough to drop whatever it is currently thinking about) than to direct one's mind to stop thinking about a particular topic. With that being said, I know that I'm also trivializing the difficulty finding a replacement topic that sufficiently demanding and important.