Musar על פסחים 233:4

We also have a statement by our sages explaining the two different introductions of the Psalms with ,לדוד מזמור or מזמור לדוד. In the former, the holy spirit came to David only after he had commenced to play the harp, whereas the words מזמור לדוד indicate that the holy spirit overcame David even before he commenced playing the harp (Pesachim 117). לדוד מזמור represented the highest spiritual achievement of man, seeing that he initiated his spiritual achievement. If man only responds to an "injection" of holy spirit the achievement is of a lower order. Malachi 3,7 exclaims: "Turn back to Me and I will turn back to you;" He asks Israel to initiate repentance. He urges us not to wait for G–d to initiate the Redemption since it would then be merely an act of love and mercy. When G–d says in Exodus 33,19 וחנותי את אשר אחון, ורחמתי את אשר ארחם, "I will be gracious to whom I choose, and I will be merciful to whoever I shall be merciful to," Shemot Rabbah 45,6 on this verse describes G–d as showing Moses all the rewards that he had stored up in His treasure chambers for the righteous. When Moses inquired who all these treasures were for, G–d answered that those people who possessed merit would receive some as a reward due them, whereas those who did not have any merits would receive a free gift from Him. Obviously, the amount of the handouts granted by G–d as unearned gifts is smaller than those gifts in store for the righteous.