What binds couples together?

I always wonder what holds some couples together. Especially when I know ones who don't seem to have anything in common. i.e. no common interests, different values, different backgrounds, and different cultures

Is it just physical attraction? I don't think so. They aren't that physically affectionate towards each other.....well, not from what I can see.

If they don't have any common interests, then what do they do when they meet? Don't they run out of things to say? Or maybe that's not important. Maybe they just like each other's company and don't need to talk that much.

Maybe it doesn't have anything to do with each individual person. It's what they become when they are a couple. Their personalities are compatible and they are better together than alone.

Or maybe it's just trust. How many people can you really trust? That you can always rely on them no matter what?

I wonder....