The upbringing of men
Children are invariably reared by their mother in the early years, regardless of the sex of the child. As we might expect, early on, the boy comes to think he is the same as his mother, and therefore that he has a right to be a mother when he grows older. Then the time comes when he is told he is not allowed to be a mother . . . because . . . he is a boy.
So, very early in life the male of our species learns that he must stand alone and unsupported. He cannot afford to get attached to anything too closely. Alienated by the central figure of his life, his mother, he learns to formulate a life plan based on self-reliance.
Perhaps this rejection by the mother plays a part in creating the vast divergence in personality between men and women.