John Gill

For thou, O Lord of hosts, God of Israel,.... As he is called in Sa2 7:26, hast revealed to thy servant; which he otherwise could not have known: saying, I will build thee an house; see Sa2 7:11, therefore hath thy servant found in his heart to pray this prayer unto thee; found his heart disposed to this service, or found freedom and boldness in him to put up this prayer to God; what encouraged and emboldened him to do it was the gracious promise of God, that he would build up his family, and establish his kingdom; or otherwise he could not have taken such liberty, and used such boldness with God in prayer, as to have requested it of him.