Anointings of David

In the books of Samuel, three anointings of David are recorded in:

I Samuel 16:12-13 (Samuel anointed David)
II Samuel 2:4 (men of Judah did the anointing)
II Samuel 5:3 (elders of Israel anointed David

When Samuel anointed David, he was a young sheep herder and Saul was the king of the Hebrews. Saul had shown that  he cared more about what the people thought rather than God. The Lord rejected Saul as king. When David was anointed with the horn of oil, the Spirit of the Lord came upon David and he certainly would need the power to do all  that God wanted him to do.

David was the youngest of the eight sons of Jesse. Three of his older brothers served in Saul's army. In the famous confrontation of Goliath, none of Saul's men had the courage to take on Goliath.  David did and got the job done, but not without some chiding from his oldest brother. Despite David's age, Saul asked to see him and David convinced him that he would take care of Goliath. Saul tried to give David his armor and sword but David did not  feel he could use them and he handled Goliath with a sling and stone:

44 And the Philistine said to David, Come to me, and I will give thy flesh unto the fowls of the air, and to the beasts of the field.45 Then said David to the Philistine, Thou comest to me with a sword, and with a spear, and with a shield: but I come to thee in the name of the Lord of hosts, the God of the armies of Israel, whom thou hast defied.46 This day will the Lord deliver thee into mine hand; and I will smite thee, and take thine head from thee; and I will give the carcases of the host of the Philistines this day unto the fowls of the air, and to the wild beasts of the earth; that all the earth may know that there is a God in Israel. 47 And all this assembly shall know that the Lord saveth not with sword and spear: for the battle is the Lord's, and he will give you into our hands. 1 Samuel 17:44-47

The Spirit of God was with David and he knew the outcome of the fight. He was bold and full of courage to defend God's people despite the negative in front of him. He killed Goliath.

After Saul died, David became King of Judah as recorded in II Samuel 2:4 when the men of the tribe of Judah anointed him as an acceptance of his kingship. The rest of the Israel made Saul's son king over Israel. There was a long war between the house of Saul and the house of David but eventually through a series of events, the elders of Israel came to David.

5 Then came all the tribes of Israel to David unto Hebron, and spake, saying, Behold, we are thy bone and thy flesh. 2 Also in time past, when Saul was king over us, thou wast he that leddest out and broughtest in Israel: and the Lord said to thee, Thou shalt feed my people Israel, and thou shalt be a captain over Israel. 3 So all the elders of Israel came to the king to Hebron; and king David made a league with them in Hebron before the Lord: and they anointed David king over Israel. II Samuel 5:1-3

Many things happened before all all of this could come together. A significant amount of time passed. God's overview of men's hearts and conditions surpasses  men's impatience in trying to arrange things for themselves. Saul failed as a king because he didn't wait on God and put his own fears into his kingship.  Confusion resulted. David became king over all of Israel as he carried out God's directions for the best benefit to Israel. Order resulted.

12 And David perceived that the Lord had established him king over Israel, and that he had exalted his kingdom for his people Israel's sake. II Samuel 5:12