Adam Clarke on 1 Samuel 25:7

Thy shepherds which were with us, we hurt them not - It is most evident that David had a claim upon Nabal, for very essential services performed to his herdmen at Carmel. He not only did them no hurt, and took none of their flocks for the supply of his necessities, but he protected them from the rapacity of others; they were a Wall unto us, said Nabal's servants, both by night and day. In those times, and to the present day, wandering hordes of Arabs, under their several chiefs, think they have a right to exact contributions of provisions, etc., wherever they come; David had done nothing of this kind, but protected them against those who would.