A critical period in a man's development is that time between the ages of 10 and 14 when a boy's identity becomes firmly established as male. Prior to that time, a boy thrives in the secure, nourishing environment of his mother. Here, as a boy, he is safe to practice manly behaviors. Then, through identification with the father (or significant male role model) the essence of masculinity is allowed to flow to the boy. If this identification process is not made, a young man's essential identity remains either connected with his mother or becomes confused.
What does it mean for a man if this identification process doesn't take place, if he stays in the sphere of influence of first his mother and then other women in his life?
It basically means that a man will be dependent on women for his "power." This can look like either a soft male who has learned to behave in a way that pleases women and therefore receives their approval. Or it can result in behaviors which push against women, even to the extent of abusive or violent behaviors. Either form of behavior reflects a deep sense of inadequacy in the man. Although any one man may predominately show one or the other of these patterns of behavior (chronically pleasing women or pushing against them), it is not unusual for a man to move from one to the other.
Please remember here that the term "Mama's Boy" is not derogatory. It is not an epitaph thrown out to indicate that someone is less than a man. It is simply a descriptive term for a man who remains in the sphere of influence of his mother and has not developed a strong masculine identity. A mama's boy is a man who has not been grounded in the masculine field.