What's his friggin problem?

What is with the breakup threats,and him still being here? We've been together for 4 years almost. He says he will leave if I mess around (did that many times and he still here) Talks about how we are going to get married and threatens not to if he is pissed off. Even once thought about it and came home without his half the rent. Can anyone read this and give me and idea? Does he even know what he wants? We've made it this far, so what's the problem and his uncertanty.