compersion

compersion

A feeling of happiness in response to knowing that someone else is experiencing joy. It is most commonly used in reference to polyamory when one's lover is experiencing happiness in a relationship or sexual encounter with another partner. My sister is polyamorous, and she says she truly feels compersion when she sees her boyfriend with one of his other lovers—but I know I would be totally jealous in that situation.
Farlex Dictionary of Idioms.
See also:
  • feel a glow of happiness
  • satisfaction
  • laugh out (of) the other side of (one's) mouth
  • laugh out of the other side of mouth
  • laugh out of the other side of one's mouth
  • laugh out of the other side of face and laugh out of the other ...
  • laugh out (of) the other side of (one's) face
  • a song in (one's) heart
  • a song in your heart
  • joie de vivre