Of all the inconveniences of flying, why is sitting next to a big person such a major issue?

Airline seats keep getting smaller. I’ve been next to people who overflow their seat onto me. I put up with it on anything shorter than transcontinental, but it does feel like an invasion of my space. I know they can’t help it, it’s a medical condition not some kind of personal failing — but on a long flight I need to have more room than that — so I ask to be reseated.

The author mentions that she booked 1st class. There’s plenty of side-to-side room in those. My issue is affording 1st class, so I’m usually shoehorned into a seat in cattle class.

That said, my biggest problem is sitting next to someone else with big shoulders (distance swimmer here). There is just no way for both of us to be comfortable.