I have to agree that its easier to feel compersion when our own needs are being met, or at the very least recognized.
For me its about security. I am secure in my husbands and my playful banter, wrestling, joking, etc etc - so when he does that with our wife I feel overwhelming compersion. I am not so secure when it comes to our sex life - so when I can hear them having sex or whatever - I dont feel jealous...I feel..something else. I have felt jealousy before - and its definitely not that, but its something all right. Not all bad, there is some compersion there its just being masked by the something else.