1. They use inferior products (very important)

2. They pay no attention to detail

3. They do not take the time to prep correctly (very important)

4. They usually employ beginners

Just a few reasons of the top of my heead.

 

A good starter book on the subject is "How to paint your car" by David Jacobs. It's available (of all places) in the J.C. Whitney catalog. One thing to keep in mind, it's a 1991 print, so some of the information (mainly to do with paint systems) is somewhat dated. The rest of the reading, from prepping to spraying, is good and informative. If you trust yourself with bodywork, don't be afraid of doing your own paintwork. Just be sure to do your homework, be it on the type and quality of equipment or paint supplies, the local and state environmental regs, or how well you get along with your neighbor - and whether he'll freak when you start spraying in your garage!