he left me for another girl after a year of us being in a situationship. why would a guy do this?

My ex and I got into a situationship a few months after we were actually dating and the situationship was a year long. About 2 days before we ended things, it appeared he got with another girl. And after we officially split, has been posting her like crazy so I’ve heard. He blocked me on pretty much everything.

I’ve asked him a few times before why we can’t be together and he would say “I want to be in a relationship with you so bad but I feel guilty that we do all of these things while not being in a relationship and I know it hurts you and me.” Stuff like that.

We never really had any issues in our situationship until I found out about the girl. When i confronted him about it he said he wanted to be with me for the longest time but he didn’t want to feel the guilt he had and knew I was hurt blah blah blah and left.

so when i guy does something like this, did they ever love the person they were with right before? or what it all just for their pleasure. what does the guy truly go through? that’s what I wanna know.