I think it's misleading to infer too much from that. Most people are nice to strangers, face to face. Maybe it's that you're suddenly a member of each other's

I think it's misleading to infer too much from that. Most people are nice to strangers, face to face. Maybe it's that you're suddenly a member of each other's in-group, and forming a basis to exclude the other person (becoming enemies) requires some initial behavioral or ideological incompatibility and enough time for it to germinate into enough of a conflict to sabotage the automatic friendly social interaction. However, just because strangers generally get along when they meet randomly, that doesn't say anything about the major problems that cause people to not get along once they know each other—incompatible ideologies, behavioral patterns that the other person can't tolerate, and so on. To pivot to another realm, a cop can be the nicest person in the world if you hang out with them at a neighborhood barbecue, and the next week they can be harassing people and ruining lives over drug possession.