Albert Barnes Commentary Psalms 21:4

"He asked life of thee, thou gavest it him, Even length of days for ever and ever." — Psalms 21:4 (ASV)

He asked life of you - An expression similar to this occurs in (Psalms 61:5–6), For you, O God, have heard my vows;...You will prolong the king’s life, and his years to many generations. The expression in both cases implies that there had been a prayer for life, as if life were in danger. The expression itself would be applicable to a time of sickness, or to danger of any kind, and here it is used doubtless in reference to the exposure of life in going into battle, or in going forth to war. In this apprehended peril he prayed that God would defend him. He earnestly sought protection as he went forth to the perils of war.

And you gave it to him - You heard and answered his prayer. He was saved from danger.

Even length of days forever and ever - You granted him more than he asked. He sought life for himself; you have not only granted that, but have granted to him the assurance that he should live in his posterity to all generations. The idea is that there would be an indefinite continuation of his race. His posterity would occupy his throne, and there would be no end to his reign thus prolonged. Beyond all his petitions and his hopes, God had given the assurance that his reign would be permanent and enduring.

We cannot suppose that he understood this as if it were a promise made to him personally, that he would live and would occupy the throne forever. Instead, the natural interpretation is that which would refer it to his posterity, and to the perpetuity of the reign of his family or descendants.

A similar promise occurs elsewhere: 2 Samuel 7:13, 2 Samuel 7:16. (Compare the notes at Psalms 18:50). It is by no means an uncommon thing that God gives us more than we asked in our prayers. The offering of prayer is not only the means of securing the blessing we asked, but also often of securing much more important blessings we did not ask.

If the expression were allowable, it might be said that the prayer suggested to the divine mind the conferring of all needed blessings. Or, it indicates such a state of mind on the part of him who prays that God takes occasion to confer blessings that were not asked. This is similar to when a request made by a child to a parent for a specific favor is followed not only by granting that favor, but also by bestowing others of which the child did not think.

The state of mind on the part of the child was such as to dispose the parent to grant much larger blessings.