Whenever some choad on the airplane tells me that he’d prefer to sit in my seat, instead of the one assigned to him, I say “You know, I’d also like to sit in my seat. But, I’d happily sit in your seat for $500 (or some amount depending on price of flight)”. That usually settles the question. Capitalism kind of sucks, but once in a while, it is fun to use it for righteousness.
Now, if it’s a parent travelling with a young child, the situation is a bit different. Here’s a pro-tip, though: If you really want to trade seats in order to sit next to your child/SO/whatever, always volunteer to trade down in order to sit next to who you want.
Anecdotally I can tell you that the number of coolers I have won from Dick’s Sporting Goods (for instance) via my free Gmail account in recent months is too damn high. Whereas I also get basically nothing to my (still free legacy Google Apps for Domains/Workspaces) Gmail account.