Eli26
Super Freak
Reasons are personal. Some people are in it strictly for a buck. Others(myself included) are in it for the joy of collecting but would rather make money when they sell than lose it. Others are hardcore fanboys and pure collectors that will never sell anything. Very few of them around here. There is no right or wrong reason. It's all in how you roll.
I seldom sell. If I do, it's because I have two of something or got something I regretted because it was the it thing, and that is rare. I am a person who doesn't know how to spend his money right.