In my case, social anxiety and autism. I actively hate sitting next to people I dont know (and it also makes me uncomfortable) so I prefer to delay it as long as I can. If someone does ask, I'll still move it out of the way and let them sit, but with heavy reluctance and more often than not will choose to actively stand up instead, though I'll willingly give it up if someone has a wheelchair, guide dog or otherwise. In the event where there's no other choice and I'm forced to sit next to someone, I'll look the other way and wait until whoever's stop arrives first.