1) They believe your image has value - The paparazzi want to sell their pictures. The more emotion they can evoke from you, the more interesting the photo. The more interesting the photo, the more value it has. The more you allow them to provoke you, the more they win. The solution to this part of the problem is to make their photos worthless by immediately publishing something better. Let me show you how.
2) They love you - Your fans adore you and want to have a piece of you that is all their own. The solution to this part of the problem is to educate your fans that taking your picture without your permission is wrong, and you don't like it. It won't stop every fan from snapping a quick photo, but is that really the problem?
Of course, there are other reasons someone might take your photo. These are the main two that require a solution right now.