Albert Barnes Commentary Psalms 78:70

"He chose David also his servant, And took him from the sheepfolds:" — Psalms 78:70 (ASV)

He chose David also his servant – He chose him that he might set him over his people as their king. The idea is that David was selected when he had no natural claim to the office, as he did not belong to a royal family and could have no claim to such a distinction. The account of this choice is contained in (1 Samuel 15:1–30).

And took him from the sheep-folds – From the humble occupation of a shepherd (1 Samuel 16:11; 2 Samuel 7:8).