Rue. So?

I seem to recall reading or talking with someone about how Jean-Jacques Rousseau theorized sympathy--i.e. why we feel sympathy when we see suffering (and, if I'm not mistaken, what the implications are for social institutions, law, etc.).

But I have no idea where in Rousseau this is. I may be completely wrong that he discussed this at all. (Philosophy was never my strong suit.) Can someone better edjmacated help me? Cursory googling didn't turn up so much.