Why? From my experience, if given the opportunity, the vast majority of people will just drag you down to the point where they begin hurting you personally including your income. Everyone has problems, needs helps, has loads of ideas, etc... few are successful.
In previous lives I was a much nicer guy, and wasted an unbelievable amount of time on people like that. Sometimes these people were even my roommates, or just friends, or business associates, or whatever. I genuinely cared about helping them though, would listen to all their bullshit, help implement their ideas, bring them into my business, or whatever. Many times, this required sacrificing my own time, ideas, ambitions, increased my stress level, etc.
These people (even the ones I lived with) would constantly tell me how I'm such an amazing guy, one of the best friends they ever met, and oh so intelligent. Once business interests faded for whatever reason though, I was no longer too important, and when going through rough times, I was basically ignored.
Low and behold though, once they believe I'm on an upswing in life, I all of a sudden become Mr. Popular again, and someone worth knowing. Then sure enough when I politely tell them to go fuck themselves, I end up being labeled the asshole in the situation. Still haven't quite figured that out. Nowadays, the only chance I'm helping someone is if a) I get a piece of the pie, and b) I believe it's an idea worth implementing.
This 1000 times. You can do all the nice things in the world for someone - help them with bills, ideas, and listen to their problems. The second they get bored or don't want to put in the effort you're yesterdays' garbage. Most of these same people will make you out to be the bad guy when they inevitably fuck you over in one way or another. To top it off, you wind up spreading yourself thinner than you already are.
Take some time for yourself. Remember this life is your life - not theirs - and that there is NOTHING wrong with being selfish. Let's face it - if you're not selfish for yourself, no one else will be.