Group of Words has an excellent story based example of why folks enter and leave guilds. Its a simple concept once explained, and reads really well.
If you’re new to Warcraft this might be helpful. If not, well you probably know all this anyway, but it is explained well. I’ve blogged it so I can find it again, and also because I think it deserves a bit of kudos.
TyphoonAndrew's - Eye of the Storm 
You should check out http://www.lookingforguild.net if you are recruiting or looking for a guild. We are the fastest growing guild recruitment website for WOW. Its 100% free.