I can't speak for everyone, but in my own case it's not that I don't enjoy helping those around me with their problems. I genuinely do enjoy helping them. The problem is that a lot of people who ask me for help fall into one of three categories at some time or another.
a) The willfully ignorant: I don't mind helping people out, but when you've called me three times about the same problem and refuse to remember anything I told you the last two times, my patience starts to wear thin.
b) The impatient: I am competent enough with computers to deal with most run-of-the-mill problems my friends and family call me about, but I am not a professional or walking encyclopedia on every possible problem. Some things take time for me to fix, and others are beyond my capabilities. You called me for help, all I'm asking for in return is a little bit of patience and understanding.
c) The know-it-all: I don't like spending hours trying to help you with something just so you can turn around and take your PC to Geeksquad when you don't like what I have to say. If you thought you knew better, why did you bother asking me in the first place. I have better things to do with my time.