Alright here me out. Someone said this in church today, and while in the context of what they were saying it actually didn't work, I thought it was accidentally quite profound. (like rumsfeld's known unknowns speech).
Love is its own reward. The euphoric sensation of being in love is enough to make it worthwhile. Therefore love doesn't need to be reciprocated by it's object in order to reciprocate, because it reciprocates itself with warm feelings.
Even when love is painful its worthwhile. There is nothing as painfully sweet as unrequited love. In fact the sensation of being in love is more potent when it's not reciprocated. The desire for the unattainable is so much stronger than the desire for the easily attained. Therefore, again, love reciprocates itself.