It is because they are emotionally disturbed and frustrated by their inability to express themselves effectively so they use their fists. If you are going through this kind of ordeal, it is better that you walk out of it as soon as he hits you.
They are insecure, scared, little boys that try and feel like men by using physical force over a weaker woman. They deserve no pity, or mercy. Anytime I've been a witness to it...it's been a one way ticket to unconsciousness for the punk involved.
All of the above are correct. Many are troubled people (although it should not be a reason for them to hit you). Some have anger issues and cannot refuse to get help, which makes them completely accountable. They are still boys. And they will be until the day they lay in their grave. They need to man up. Hope this helps.