If you are going to bring Christian theology into this: God created Man with free will. He created him neither to be good nor bad, but to choose. Absent any teaching, Man will usually choose selfishness.
Without theology: Man is an animal capable of higher order thinking. In his natural state, he will care for himself. He can learn to empathize with others. Some men are capable only of understanding that their own selfish needs can be met by acting in ways that care for others. However, the starting point for man is selfishness. One way or the other, that selfishness has to be learned away.
The above is a postulate, just as your opposing POV is a postulate. If a reader accepts my postulate, your whole point collapses. If he accepts yours, your argument may stand. May.
I submit my postulate for consideration, confident that most people already or will come to agree with it, and I will move on.