Why do people stay in relationships that do not fufil them?

Recently, I’ve been overly frustrated with a person who- despite any relationship staus he may be in with someone else: always winds up “coming back” to me.

I have known this individual for about 7 years, and have seen him on a regular basis due to now living in the same town: and being involved with the same sports team for the past 3 years.

Initially, he tried making advances at me when he was unhappily dating his baby’s mother. I’m no homewrecker, and not an idiot, and evaded that situation. Eventually they ended up seperating and he persued me for more than a year.
We sort of had something going on , eventually for varous reasons, the situation sort of just died.

He started seeing someone else.
He is still with that someone else.
We’ve maintained friendship with no other romantic/more than friend connotation.

Two days ago he sent me a message how he cant get me out of his mind and all this non sense about potentially being his wife…
He thought he coulg just stop thinking about me on a more than friend level so on and so forth…
Again. not a homewrecker,
quickly reminded him of his current relationship status, and told him he can’t have both:
when in reality…. why would I even
give this guy a chance, when he clearly has displayed his true colors….
I don’t know how much depth of personality his girlfriend has, we’ve never spoke- however she is very attractive. I just can’t grasp the idea and unfairness of this all.

and it just keeps going from there.

So after that ridiculously long story, My real question is why do people do these sorts of things? Why would you be in a committed relationship if youre not onehundred percent down for that person.In my case, the man in question is in his late 20’s. Its not like he’s in highschool… sure, everyone likes to have someone there…but really? Anybody else run into situations like this? It seems to happen to me a lot.