Perhaps we're just thinking too hard about matters.
For whatever reason, a person feels good for helping others. They do not necessarily do it just to "feel that good feeling," although it certainly doesn't hurt that they feel that way and it might spur them on to do the good deeds.
We do ANYTHING in life because we have a desire -- an unmet need/want -- and fulfilling the desire helps satisfy the need. Why is the desire there in the place? I don't know the answer to that.
Are some of us simply biologically wired that way? Maybe, maybe not.
Is there something metaphysical going on? Maybe, maybe not.
Taking it down to that level seems to not answer any questions. It's rather like the issue of free will: Regardless of whether I actually have it, I *feel* as if I do, and so I choose to do and say things that possess meaning (according to my definition of such).