I broke up with him, why does it hurt?

I met a guy recently and we hit it off big time. After a couple of weeks, I noticed that he wasn't as nice as he was when we first met. He'd snap at me, complain if I didn't answer his call or texts, and was just pretty much self-centered. I have a very strong "I don't take anyone's crap" philosophy and I figured that if things went sour after just 1 month, they sure wouldn't get any better. So why do I miss him and why does it hurt?