What makes men become abusive in relationships after marriage/few years?

I'm wondering if people can be honest here and say why it's so often the case that before marriage a guy would never hit his girlfriend/wife but more often than not after marriage they hit their wives, abuse them physically and mentally, cheat, leave the family alone. Be honest, this question is not a way to offend anyone. I want genuine opinions and thoughts brought up here.
What makes men become abusive in relationships after marriage/few years?
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