If I spot a couple or a family being separated and it does not cause me undue inconvenience, I voluntarily offer to swap seats with them. I must have offered about 3-4 seat swaps this year. I find that most people do not want to ask but happily take the offer of a swap if someone volunteers.

In case some generous soul thinks I'm some kind of a nice person, I'd like to emphasise that it sometimes benefits me to do that - a happier, and therefore quieter, kid for not being separated from mother/father, and less movement in the cabin for a couple not being separated and subsequent chatter by my seat between the separated person and his/her a 'visitor' companion being reduced. So it's often a win-win!

However I totally see that some people choose a specific seat for a reason. Some people even do so because of an injury etc and I would never assume that one seat is the equivalent of another. I also wouldn't ask to swap seats except in an extenuating circumstance. Being seated away from my husband is definitely not what I consider an extenuating circumstance, considering we sometimes even travel in a separate cabin or different routes to get to the same place. But we're a pretty independent couple and have our own travel habits. I sympathise with those who want to be seated together.