1 Pictures reverse the image we see in a Mirror
Because most humans have imperfect faces , where the left side isn't identical to the right side, they feel awkward when they see an inverted picture of their faces (See also why some people look bad in pictures)
2 Our features are projected on a 2D image
While the human Eye sees a 3D picture of people images project those images on 2D surfaces. Some people don't look good in the 2D form (See also why some people hate to appear on pictures).
3 Pictures show how we look from one angle
Most people don't look good from all angles. Pictures usually focus on one angle and as a result some people might hate to see their pictures taken.
4 The brain alters the perception of pictures
The brain alters the perception of the images we see depends on our beliefs. If a person thinks he is ugly then he will see an ugly image when looking at his picture.
5 We are unfamiliar with our camera photos
People feel more familiar with the images they see often in the mirror but because pictures aren't taken everyday a person can easily find his pictures looking unfamiliar (See why we like familiarity).
6 They are not photogenic
Some people look bad on pictures even if they look good in real life. Those people usually hate to see pictures of themselves as they make them look bad.
7 They are used to a certain level of lighting
People are usually used to a certain level of lighting when they see themselves in their preferred mirrors. Because Camera's change that people feel unfamiliar when they see their pictures.
8 We want to look more attractive
Many people avoid taking pictures of themselves because they want to look more attractive than their photos. Those people hate their photos because they make them feel less attractive.