Saint Bernard of Clairvaux

Love is sufficient of itself; it gives pleasure by itself and because of itself. It is its own merit, its own reward. Love looks for no cause outside itself, no effect beyond itself. Its profit lies in the practice. Of all the movements, sensations and feelings of the soul, love is the only one in which the creature can respond to the Creator and make some sort of similar return however unequal though it be. For when God loves, all he desires is to be loved in return. The sole purpose of his love is to be loved, int he knowledge that those who love him are made happy by their love of him. 

-- Saint Bernard of Clairvaux from a sermon