Kabbalah zu Schemuel II 23:23

but he did not go over to them. Even though he did not join them and was not included in the count, "David set him over his guard" (Ibid.) because he was never separated from the tablet of his heart, because they are inseparable. David gave his heart to him, but he did not (do the same) to David. Because by the hymns and praises together with the kindness that the moon shows for the sun, she draws him (the sun) to reside with her. This is what the verse, "And David set him over his guard," means.