Why?

Why do people commit to a relationship that makes them unhappy? I really don't understand. I'm not talking about long term relationships, but new relationships where the couple only know each other for a short time. Often, I hear things like " It's almost impossible for us to have a discussion"; " I just don't like how he doesn't think for me" etc. Or there's not enough respect between the couple. Is there an explanation behind this?

From what I gathered from acquaintances, what made them stay in the relationship is the notion that they have someone that's staying in their life and the person will look after them. I know I sound stupid but I really don't get it. How can someone have the ability to take care of you if they can't even make you happy? I have the belief that a significant other should be able to become your best friend or at least a good friend after stripping off the other romantic aspects. Maybe it's the sex and chemistry that's keeping them? But not very often. Some of them don't have a partner that is physically attractive to them, one they don't have the urge to drag them to bed. This is what really baffles me. If they don't have the magical powers to make you wanna be naked with them all the time, and you can't have a proper discussion with them, and they don't think for you or don't respect you enough, what's the catch?

There must be something that I'm missing out.