Deuteronomy 31:1

Loth to part (we say) bids oft farewell. Moses does so to the children of Israel: not because he was loth to go to God, but because he was loth to leave them, fearing that when he had left them they would leave God. He had finished what he had to say to them by way of counsel and exhortation: here he calls them together to give them a word of encouragement, especially with reference to the wars of Canaan, in which they were now to engage. It was a discouragement to them that Moses was to be removed at a time when he could so ill be spared: though Joshua was continued to fight for them in the valley, they would want Moses to intercede for them on the hill, as he did, Exo 17:10. But there is no remedy: Moses can no more go out and come in, Deu 31:2. Not that he was disabled by any decay either of body or mind; for his natural force was not abated, Deu 24:7. But he cannot any longer discharge his office; for, 1. He is 120 years old, and it is time for him to think of resigning his honour and returning to his rest. He that had arrived at so great an age then, when seventy or eighty was the ordinary stint, as appears by the prayer of Moses (Psa 90:10), might well think that he had accomplished as a hireling his day. 2.

— Matthew Henry's Concise Commentary (public domain)