A big source of externalities I’m not sure I’ve heard mentioned as such: people caring about one another.
For a random person, I’d guess the unaccounted for effects on other people from their making different private decisions about their lives while being cared about by numerous family and friends might be at least comparable to those from marginally contributing to air pollution or city density or traffic congestion.
Wicked good point.
another neglected source of externalities is prejudice (which is a form of "care" I guess). A gay couple might have large negative externalities on nearby bigots if they walk down the street holding hands (I think this example is a nice counterexample to the idea it's always good for the government to internalize the cost of externalities).