In terms of the law, you had no obligation to make the trade. Only if safety is a concern can a seat switch be legally required. Certain airline policies also allow flight attendants to rearrange passengers in cases of unsafe weight distribution. Otherwise your seat is like your very own nation-state: You determine its trade policy.

You had every right to sit tight. That said, some passengers—seven footers, anxious aerophobes, and elders with arthritic knees—have compelling reasons for seeking a different perch. And it never hurts to practice a little in-flight kindness and agree to a swap. You may be less comfortable sitting scrunched between two people. But by the time the plane lands, you’ll have earned something more valuable than frequent flier miles: good karma.