Mom commonly verbally emasculated Dad. Dad commonly verbally abused Mom. They taught him a thousand subtle ways to tear people down. In the process he perceives that they are tearing him down. He grows up in an environment that tears people down, and in that environment he is always afraid, always unsure of himself.
Learned behavior is a door.
His home was not a religious home. Pornography was openly used by Dad. Dad encouraged him to develop the same desires and attitudes. Peer influence is a door. The teenage world is full of pornography.