King David was popular with his people for many reasons 510-2 - 9 - 2

King David was popular with his people due to his early bravery, his servant’s attitude, his loyalty to King Saul and his many victories in battle.

As a lad, David had a servant’s humble attitude and didn't consider above himself being a shepherd. A thousand years later a humble Jesus would step out of His glory, Philippians 2:6-7, "6 Who, being in the form of God, thought it not robbery to be equal with God: 7 But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men:"

Early on David exhibited great bravery as he protected his herd of sheep from both a lion and a bear. While David was still a boy the Philistines gathered their armies to battle against King Saul and his army. The giant Goliath taunted King Saul with a one-on-one challenge against him. King Saul stayed with his men and the stalemate continues for an extended time. Then enter David.

David had been sent to carry food to his brothers and said this about Goliath’s challenge. 1 Samuel 17:26, “And David spake to the men that stood by him, saying, What shall be done to the man that killeth this Philistine, and taketh away the reproach from Israel? for who is this uncircumcised Philistine, that he should defy the armies of the living God?” 1 Samuel 17:36, “Thy servant slew both the lion and the bear: and this uncircumcised Philistine shall be as one of them, seeing he hath defied the armies of the living God.”

David met Goliath armed only with a sling and five stones and The Whole Armor of God. David was so confident in his victory through God, he was prepared to battle Goliath and Goliath’s four brothers who also were giants.

King Saul had lost the favor of God and was rather unbalanced and David often play his harp and sang to him to soothe him. As David became more popular with the people, King Saul was jealous of David and sought to kill him. Nevertheless, David was always loyal to King Saul as God’s anointed.

Later as King, David was an outstanding warrior with a very loyal following of his mighty men. David was in fellowship with God and as such enjoyed one victory after another. 
Sadly, King David had two major failings. The first being the sin of the flesh. In addition to the taking of many wives was his adultery with Bathsheba and arranging the murder of her husband. The second failing by King David committed the sin of pride by initiating a census.

David repented of his sins and was forgiven by God and was still a man after God’s heart. Nevertheless, there was a price to be paid. 70,000 died because of the census and David’s house was dysfunctional with incest and his sons rebelling against him.

Regardless of his shortcomings, King David is considered the greatest King of Israel until the reign of King Jesus.