I'm pretty conflicted myself, so I can relate to others in similar situations. It hasn't always (or rather, never has) been to my benefit, but there's some, overwhelming part of me that is achingly wanting to help those in need even if I know I'm not going to get any favors in return. When I'm spurned, or when my efforts backfire, I retreat into a more cynical viewpoint that has me considering more Machiavellian ideals. The more INTJ tendencies behind my person come back to scold me.
But I'm beginning to think that maybe this is not so much an innate thing as it is a self-esteem thing. I like the idea of being in control, of having people depend on me; it appeals to the leader in me. You're not going to go hang out with the independent, powerful, confident people if you're looking to have someone lean on you as a reassurance that you're a strong, capable person. No way. It's the confident, extroverted types that always get us in trouble because they take advantage of INFJ's sensitivity and incapability of being as cunning or as shrewd as the other types.
It's a pretty sinister theory, but maybe, deep down, we do it for selfish reasons. It makes us feel needed rather than vulnerable in a world thatoften doesn't understand our ways.