When we play, we usually have our own PC-Playing Character (let's leave aside for a moment games which don't give you a PC but a community or shared/rotating PCs etc..).
Of course, we do care about our own PC (in different ways - depending on our approach and the game style - from power-play to "let's see how this will end").
But what about other players' PCs? Do you care about them? And how?
Do you enjoy the story of another PC more than your own, sometimes? Why? Because you're just a witness or play a minor role in it? Or for other reasons?
(and maybe: how much is this influenced by your relationship with the other Player?)
It's maybe a bit vague, I'm more brainstorming than making a point here...