My thoughts on why I would want to mod something very popular is because I would almost immediately have a following if my product is good. Also, I'd be able to create a mod that has functionality that I desire, and I'd get the satisfaction out of using it. And lastly, I don't know if this is always true, but it certainly makes you feel more elite changing the behavior of someone else's code rather than my own. There may be other reasons I haven't mentioned.