Love to love the lovers who love to love the love.
I do think that compersion is real, but it takes a solid basis of communication, time spent caring for the primary relationship, and checking in to keep compersion going for any length of time. There's a big difference between everything running smoothly and you feeling really happy for your partner's connections and there being a bump in the road and that happiness sticking around!
To me, the better that conversations, the more self inventory, and the more honesty, the closer compersion comes to the surface. Good diet, sleep, exercise, self love/care in general also make a big difference!