Piper quotes Augustine twice:
Who has it in his power to have such a motive present to his mind that his will shall be influenced to believe? Who can welcome in his mind something which does not give him delight? But who has it in his power to ensure that something that will delight him will turn up. Or that he will take delight in what turns up? If those things delight us which serve our advancement towards God, that is due not to our own whim or industry or meritorious works, but to the inspiration of God and to the grace which he bestows
He defines it clearly in On Christian Doctrine (III, x, 16). "I call 'charity' [i.e., love for God] the motion of the soul toward the enjoyment of God for His own sake, and the enjoyment of one's self and of one's neighbor for the sake of God." Loving God is always conceived of essentially as delighting in God and in anything else for his sake.
http://www.desiringgod.org/ResourceLibrary/Biographies/1474_The_Swan_is_Not_Silent/