I have started reading "No more Mr Nice Guy." Robx, I have nominated you to be one of my "Safe Male Friends" I am assuming you are male. If not, you put on one hell of a show. So, I am suppose to say what from my childhood helped in giving me the "Nice Guy Syndrome", which I am not yet convinced I have.
Really, my childhood was pretty normal. I actually had a pretty good childhood. But my parents did live a lot of their lives through us children. They were always bragging about our accomplishments to family or friends. It was something I really hated. Not sure what that would have to do with my current sitch, but the book said to share it with you.
Just thinking out loud here. Could it be that you got used to getting praise for performing well and for someone else's pleasure, vicarious ego boost and amusement? That you learned to stuff it and do what made you look like a great kid worth bragging about rather than what you perhaps genuinely wanted to do or be?