The real reason is that dudes who do this are dickheads with tiny peckers who hate themselves so much and feel so weak internally that they lash out like little babies at the easiest target, who is often his girlfriend, wife or child. Then because he's such a coward he won't even take the blame for his actions.
He will never stop hitting you. If you don't leave him he may even put you in the hospital or worse yet, in the ground. If you live with him, pack up immediately and go somewhere else. Go to a friends. A relative. Anywhere. Just go! Do not fall for his b.s. anymore.
If you don't follow my advice, which I sincerely hope you do, leave documentation like photos of your bruises in a safe place so that should the worst happen, at least he can be prosecuted.