It has nothing to do with not being able to stand a few hours' separation. Except for retired couples with no individual interests, we all have that much or more time apart on a regular basis. It's that the comfort level of rubbing up against someone for several hours, leaning into them, sharing an armrest with them, stepping over them, etc., etc., is much higher when it's your SO than when it's a total (and perhaps smelly or otherwise annoying, it's hard to know ahead of time) stranger.