I have been pondering this bit about the “mercenary heart” all afternoon.
The crux of it being that love itself is a uniquely self-less action.
I do not believe this is the same as “instincts,” which are natural.
Love is natural too.
But love is also profoundly human, and therefore theological.
A love enjoying their lover is no bribe, is it?
We are afraid that Heaven is a bribe, and that if we make it our goal we shall no longer be disinterested. It is not so. Heaven offers nothing that a mercenary soul can desire. It is safe to tell the pure in heart that they shall see God, for only the pure in heart want to. There are rewards that do not sully motives. A man’s love for a woman is not mercenary because he wants to marry her, nor his love for poetry mercenary because he wants to read it, nor his love of exercise less disinterested because he wants to run and leap and walk. Love, by definition, seeks to enjoy its object.
-C.S. Lewis, The Problem Of Pain (Amazon)