Charles Ellicott Commentary 1 Samuel 16:21

"And David came to Saul, and stood before him: and he loved him greatly; and he became his armorbearer." — 1 Samuel 16:21 (ASV)

And he became his armor-bearer.—But probably only for a very short time. David returned, we should conclude, to Samuel, whose pupil and friend we know he was. The seer was watching over the young man with a view to his lofty destiny.

Saul, apparently from his question in 1 Samuel 17:55, “Whose son is this youth?” had forgotten all about him. There is no “note of time,” so we are not able to determine how long a period had elapsed between the events narrated in this chapter and the combat with the Philistines told in 1 Samuel 17. It is, however, likely that the king’s malady, which was making rapid progress in this period of his reign, had already obscured his once powerful mind; his memory for the past was likely to have been treacherous.