There are a lot of little reasons. Yet there is only one fundamental reason: sex.
A woman's bottom line in a relationship: Don't cheat on me.
A man's bottom line in a relationship: Fuck me. Feed me. Don't yell at me.
Both sexes want more; both sexes absolutely will not accept less.
Here is the truth about relationships:
Women seek hope through change. Men hope things won’t change. Change is an unknown because one cannot definitively guarantee the result. Men hate unknowns. A man can't plan a reaction if he can't anticipate the action.
A man has a certain set of comfortable "reactions." He knows defense. He knows offense. He's willing to defend; he's willing to offend. Any action that requires a reaction outside his set is avoided.
When a man is in an unsatisfying sexual relationship, (and no need to assign blame because it doesn't matter who is at fault. It matters that the relationship isn't working) he will not tolerate the strain. He knows a change is imminent. So, he'll create a change that suits him. He'll be the agent of change. But he'll change to a situation he knows. He'll return to the actions of courtship. He will revert to the hunt for a compatible mate.
Men will always choose the actor (the agent of the action) who offers the least resistance. Men only want friction during the fuck.