I've had a woman come up to me at Marylebone late at night once and say something like "I don't know you, but do you mind if we have a quick chat so those drunk lads over there leave me alone?" - this is actually fairly terrifying to me as I'm terrible at smalltalk, but I did my best! She got on a different coach when the train arrived (it was a 3-car - she got in one, the lads in another and me in the third one), perhaps she had picked up on me looking scared!
Maybe I just look trustworthy (or just big enough to knock said lads over if they did do anything)
Hah that reminds me. Many years back I was flying from Vancouver to London and got chatting to a girl in the check in queue. She was quite friendly but horrendously nervous about flying. So we had a drink in the airport bar, I reassured her and we went our separate ways as the flight was called as we were seated at different parts of the plane.
Note I was working in Vancouver at the time and was home just for a weekend for a family event, so really wanted to sleep on the plane - which I'm not good at. And to my utter joy, when the plane doors were closed I seemed to have the only empty seat next to me on the flight - and in a 2-4-2 configuration it was even better so I could stretch out.
About two minutes after the seatbelt sights went off, suddenly the girl from check in appeared and plonked herself in the seat next to me, saying the guy next to her was a weirdo and she'd rather sit next to someone she trusted. So I was bolt upright for the whole flight, and she wanted to talk anyway most of the 10hr journey to forget about her fear of flying, so I had no sleep overnight, and I had to go to a party on arrival...
Luckily I was in my early 20s at the time so it was a bit easier to deal with, but nowadays id be well miffed

It's great to look trustworthy I guess, but losing my double seat was a bummer (but I guess it was good to make someone's flight a lot better).