What a beautiful story, the anointing of David. Anointing of oil was symbolic of setting a person apart for God’s service. “…from that day on the Spirit of the Lord came upon David in power”.
We too have been anointed.
2 Cor 1:21-22
Now it is God who makes both us and you stand firm in Christ. He anointed us, set his seal of ownership on us, and put his Spirit in our hearts as a deposit, guaranteeing what is to come.
David was anointed king, yet he had to wait to be king, even though he was empowered by the Lord’s Spirit. Think about all the things that David learned while waiting. How often does God choose to have us wait? Do we use our waiting time wisely?

Why did God allow an evil spirit to torment Saul? (see 1 Kings 22:19-23 and Job 1:6-12)
At this point in time, Saul did not know that David had been anointed as king.

The armies of the Philistines and Israelites were camped opposite each other with a valley in between with steep walls on each side.  Often armies would send their strongest to battle to declare the winner to save the loss of many lives.  This is why Goliath (obviously a mighty warrior) was the champion for the Philistines.

When David related his success with a bear and a lion, we imagined that David might have struck the animal with his slingshot as the animal was leaving with the sheep in his mouth.  Then when the animal turned after being struck, David struck back and killed the wild animal in order to save the sheep. Now, this scenario doesn't seem humanly possible.  However, we are told that the spirit of the Lord came upon David in power on the day that he was anointed by Samuel.  Was David given the same physical strength that Samson was given?

What faith David had! He acknowledged that his deliverance from wild animals came from God and he had no doubt that God would deliver him from Goliath.

How is it with our faith? Do we trust God to deliver us from our “Goliaths”? Remember the entry from yesterday?
1Ch 5:20
And when they prevailed over them, the Hagrites and all who were with them were given into their hands, for they cried out to God in the battle, and he granted their urgent plea because they trusted in him. 

According to Acts 13:22 David was a man after God's own heart.
He raised up David to them to be their king; to whom He also witnessed and said, I have found David the son of Jesse to be a man after My own heart, who shall fulfill all My will. 
In order to be a man (or woman) after God's own heart, we must do His will.  Just another example of complete obedience to Him.