He shall love as long as the sun and moon endure, from one generation to another.
The psalmist prays for God's blessing on the king here, that he will be the guarantor of justice and peace. And the prayer is that he will live forever, or we might say: that justice and peace will prevail for ever.
Sometimes it is easy to forget that justice and peace will prevail. Sometimes the darkness prevails, there is little light, and forever seems over.
But we who are spiritual seekers usually do not have to look far to be reminded that light pierces the darkness, that love trumps fear and hope cannot be taken away.
(I just looked up at the line from Ps. 72 and saw the Freudian slip...do you see it?)