I once had to sit next to this fat guy with really sweaty pits. He was so big that every time he relaxed, his sweaty pit pushed up against my shoulder. I had to keep telling him to stay off me. Disgusting.
A second seat would have been appropriate there.
I once had to sit on a CRJ next to an obese teen girl. She was nice but was so big we couldn’t put up the armrest between us and her fat rolls landed in my lap.
Fortunately one seat was available when everyone finished boarding and I jumped on it complaining that I needed to stretch my legs. I guess I’m nice like that but I’m sure she knew why.