The ways in which they are difficult, or sweet, or angry, or tired, or loving were made especially for you. You were equipped by God to take care of these specific beings, even if it may not seem like it.
He did something wrong, multiple things wrong, yet once he repents, God forgives him. And more than that God forgets it. He chooses him to be anointed, to be king, to be a mighty warrior, a writer of Psalms, even though he knew David would commit such a horrible series of acts. And he calls us to do the same: forgive and accept forgiveness.