Because it is human nature and the mind is the most amazing creation known. Thoughts are simply our true being trying to absorb and process how we feel about all the trillions of pieces of information we recieve through our seeing and hearing. Everyone thinks thoughts they shouldn't because we are designed to. Anyway, what you think is a thought that you shouldn't be thinking isn't a bad thought for someone else. Chill out and think about everything you want to and then decide what is right for you and discard the bad thoughts like cleaning out a computer's files.