Over the past few years I've traveled quite a bit, Sitting next to a stranger on a plane for hours at a time is an inevitable part of this travel gig, and I've noticed that as I've gotten older, I am less and less likely to talk to other people on the plane. To the point where talking to my seatmate is actually an exception for me. The last several trips I've taken, there were zero words exchanged between myself and the person to my right or left. (Except on my way to Israel, when I sat next to a guy who finished my food for me.)
If someone tries to talk to me, I am friendly and cordial, though I will usually try to give some indication that I'm not open to a long conversation, by either having a book out or headphones in my ears. But I would much prefer not to talk at all. It's not that I'm averse to talking. I think I just start panicking that it may never end if it gets started. We've all been in those scenarios where you say one thing to someone else on a plane or in a cab and then suddenly they feel compelled to talk and fill every moment of silence with conversation. So sometimes my fear of small talk overrides social graces, because it's best to just not open the possibility of conversation in the off chance that I might get trapped in conversation for the whole plane ride. Which, minor as it sounds, really is difficult and taxing for an introvert. Especially when plane rides to the opposite coast can be 5+ hours.
I'm also usually pretty exhausted when I'm flying. Chances are I've spent the previous 24 hours trying to pack, finish up any work I would have done during my travel time, get in some quality time with my kids, and get the house ready for me being gone. It's rare that I get a full night's sleep before a big trip, and usually I have some work to do (or a talk to write) while I'm on the plane. I also really appreciate that time as a rare quiet space where I can catch up on a book or some podcasts.
As an introvert, small talk can be really awkward. Small talk in a confined space with no escape route or designated end time can be really intimidating. And I learned the hard way, back when I was a therapist, that it's an occupation you should NEVER admit to on a plane, unless you want to spend 3 hours hearing someone talk about their recent divorce or issues with their dad.
The one exception for me is if I'm traveling with friends, and then I'm totally great with being chatty. That's the introvert's paradox. We love talking if it's deep and meaningful and with people we trust. I got to sit with Heather and Rebecca on the way to Peru and they made that 10 hour flight so much fun. We laughed, we cried, we schemed, we overshared. It was awesome.