How Do We Choose the One We're With?

So many times, working at a jewelry store, where couples come in all the time, I wonder what draws these people together.
There have been many cases where I think, "You two are a couple? How the hell did that happen?!" Either the man seems more attractive than the woman, or the woman more attractive than the man. Or one is personable while the other seems upset or unfriendly.

What is it exactly, that leads people to a relationship with a particular person?

There is something that I think I'm missing here. I understand that personality is the ultimate deciding factor in whether or not people stay with each other... but sometimes I wonder...

My only guess is that their relationship is based on previous years of friendship and then they just ended up together. Or that the other person lacks the confidence to believe they could be with someone better (and I don't mean someone more attractive... I mean someone who will treat them better). For example, I was helping a couple the other day. They were looking for his wedding band. He told his woman what he liked and she told him that what he liked wasn't an option. She kept pushing him to like a different kind of band. He continued to disagree... for awhile... but then finally gave in to her consistent nagging. I just sat there thinking... why are you putting up with this?! You're gonna have to deal with this in every future decision that you want to make!

Another example, a couple comes in. The woman is begging (or what seemed more like telling) he boyfriend to consider something and if he didn't, she wasn't going to be happy. She wouldn't even acknowledge my existence when I said hello. He looked irritated by her and said hello to me. I asked him how I could help. She got pissed off and walked away.

I understand that people have bad days... but come on... really?

I'm honestly curious... how do people get together and how do they end up staying together even when it's a bad partnership?
Is it comfort? Is it a lack of caring? Is it because they people in the relationship are willing to put up with the other persons shit because they really do care about them?


I just don't know.