got stuck next to a very obese person on an overseas flight. It was a horrible experience. I spent the first couple days of my trip with neck pain from the awkward angle I had to sit for so many hours. I know this isn't true of all overweight people, but this particular one also smelled pretty bad.

On the flight back, I did pay to upgrade my seat to avoid another situation like that, but I disagree completely with you that I have some sort of responsibility to do that. He was the one who was taking more than his fair share of space, not me. I feel sympathy for him because I'm sure he was uncomfortable as well, but he had some level of choice in the situation; I had none.