I travel almost exclusively for pleasure and one of the things I love about it is meeting and speaking to people from around the world.
However, on a plane I really just want to catch up on movies, read, and stare blankly. It's one of the few times I have to veg out and kind of shut down.
And I agree with others who have been "burned" a few times by ending up listening to stories that I just don't care about.
In a cafe I can find a way to gracefully bow out of an unwanted conversation. But in an airline seat on a long flight it's obviously a bit more challenging.
I hate the idea of hurting someone's feelings so I typically feign interest while wishing my heart would stop beating.