Nathan said to the king - In this case he gave his judgment as a pious and prudent man, not as a prophet; for the prophets were not always under a Divine afflatus; it was only at select times they were thus honored.
For the Lord is with thee - Thou hast his blessing in all that thou doest, and this pious design of thine will most certainly meet with his approbation.

And Nathan said to the king, go, do till that is in thine heart,.... He perceived it was in his heart to build an house for God; he knew an house was to be built at one time or another, by some person or another; he knew it was a good work, and fit for a king to do, and might think this was a proper time any, he being at leisure, and therefore encouraged him to it: but inasmuch as the time when and the person by whom this was to be built were not pointed out particularly in the word of God, David and he should have consulted the Lord about it; in this they erred, and for which they were tacitly reproved; for, as the event shows, this was not the time when, nor David the person by whom, it was to be built. Nathan said this as a pious and good man, in a private capacity, not as a prophet, or under a spirit of prophecy; for prophets did not always speak under such an influence, but, as private men, said some things ignorantly and through mistake; see 1-Samuel 16:6,
for the Lord is with thee; prospering and succeeding him in all he undertook, giving him rest from all his enemies; and he might think that this motion he now made of building an house was from the Lord; the Targum is,"the Word of the Lord shall be for thine help,''
or thine helper, and shall assist thee in this work. David being thus encouraged by the prophet, his thoughts were more employed about it, and he was resolute and eager to perform it; and now it was he penned the hundred thirty second psalm, in which he expresses his oath and vow to find a place to build on, Psalm 132:1.

Nathan said to the king, Go, do all that is in thine heart--The piety of the design commended it to the prophet's mind, and he gave his hasty approval and encouragement to the royal plans. The prophets, when following the impulse of their own feelings, or forming conjectural opinions, fell into frequent mistakes. (See on 1-Samuel 16:6; 2-Kings 4:27).

Nathan said - Pursue thy intentions, and build an house for the ark. The design being pious and the thing not forbidden by God, Nathan hastily approves it, before he had consulted God about it, as both he and David ought to have done in a matter of so great moment. And therefore Nathan meets with this rebuke, that he is forced to acknowledge his error, and recant it. For the holy prophets did not speak all things by prophetic inspiration, but some things by an human spirit.