A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
1 Kings (1 Samuel) 16:1
sa1 16:1SAMUEL SENT BY GOD TO BETHLEHEM. (Sa1 16:1-10)
the Lord said unto Samuel, How long wilt thou mourn for Saul--Samuel's grief on account of Saul's rejection, accompanied, doubtless, by earnest prayers for his restitution, showed the amiable feelings of the man; but they were at variance with his public duty as a prophet. The declared purpose of God to transfer the kingdom of Israel into other hands than Saul's was not an angry menace, but a fixed and immutable decree; so that Samuel ought to have sooner submitted to the peremptory manifestation of the divine will. But to leave him no longer room to doubt of its being unalterable, he was sent on a private mission to anoint a successor to Saul (see on Sa1 10:1). The immediate designation of a king was of the greatest importance for the interests of the nation in the event of Saul's death, which, to this time, was dreaded; it would establish David's title and comfort the minds of Samuel and other good men with a right settlement, whatever contingency might happen.
I have provided me a king--The language is remarkable, and intimates a difference between this and the former king. Saul was the people's choice, the fruit of their wayward and sinful desires for their own honor and aggrandizement. The next was to be a king who would consult the divine glory, and selected from that tribe to which the pre-eminence had been early promised (Gen 49:10).
1 Kings (1 Samuel) 16:2
sa1 16:2How can I go?--This is another instance of human infirmity in Samuel. Since God had sent him on this mission, He would protect him in the execution.
I am come to sacrifice--It seems to have been customary with Samuel to do this in the different circuits to which he went, that he might encourage the worship of God.
1 Kings (1 Samuel) 16:3
sa1 16:3call Jesse to the sacrifice--that is, the social feast that followed the peace offering. Samuel, being the offerer, had a right to invite any guest he pleased.
1 Kings (1 Samuel) 16:4
sa1 16:4the elders of the town trembled at his coming--Beth-lehem was an obscure town, and not within the usual circuit of the judge. The elders were naturally apprehensive, therefore, that his arrival was occasioned by some extraordinary reason, and that it might entail evil upon their town, in consequence of the estrangement between Samuel and the king.
1 Kings (1 Samuel) 16:5
sa1 16:5sanctify yourselves--by the preparations described (Exo 19:14-15). The elders were to sanctify themselves. Samuel himself took the greatest care in the sanctification of Jesse's family. Some, however, think that the former were invited only to join in the sacrifice, while the family of Jesse were invited by themselves to the subsequent feast.
1 Kings (1 Samuel) 16:6
sa1 16:6Samuel said, Surely the Lord's anointed is before him--Here Samuel, in consequence of taking his impressions from the external appearance, falls into the same error as formerly (Sa1 10:24).
1 Kings (1 Samuel) 16:11
sa1 16:11HE ANOINTS DAVID. (Sa1 16:11-14)
There remaineth yet the youngest, and, behold, he keepeth the sheep--Jesse having evidently no idea of David's wisdom and bravery, spoke of him as the most unfit. God, in His providence, so ordered it, that the appointment of David might the more clearly appear to be a divine purpose, and not the design either of Samuel or Jesse. David having not been sanctified with the rest of his family, it is probable that he returned to his pastoral duties the moment the special business on which he had been summoned was done.
1 Kings (1 Samuel) 16:12
sa1 16:12he was ruddy, &c.--JOSEPHUS says that David was ten, while most modern commentators are of the opinion that he must have been fifteen years of age.
1 Kings (1 Samuel) 16:13
sa1 16:13Then Samuel took the horn of oil, and anointed him--This transaction must have been strictly private.
1 Kings (1 Samuel) 16:14
sa1 16:14The Spirit of the Lord departed from Saul, and an evil spirit from the Lord troubled him--His own gloomy reflections, the consciousness that he had not acted up to the character of an Israelitish king, the loss of his throne, and the extinction of his royal house, made him jealous, irritable, vindictive, and subject to fits of morbid melancholy.
1 Kings (1 Samuel) 16:19
sa1 16:19Saul sent messengers unto Jesse, and said, Send me David--In the East the command of a king is imperative; and Jesse, however reluctant and alarmed, had no alternative but to comply.
1 Kings (1 Samuel) 16:20
sa1 16:20Jesse took an ass laden with bread, and a bottle of wine, and a kid, and sent them . . . unto Saul--as a token of homage and respect.
1 Kings (1 Samuel) 16:21
sa1 16:21David came to Saul--Providence thus prepared David for his destiny, by placing him in a way to become acquainted with the manners of the court, the business of government, and the general state of the kingdom.
became his armour-bearer--This choice, as being an expression of the king's partiality, shows how honorable the office was held to be.
1 Kings (1 Samuel) 16:23
sa1 16:23David took an harp, and played with his hand: so Saul was refreshed, and was well--The ancients believed that music had a mysterious influence in healing mental disorders.