Saul was afraid of David, because the Lord was with David but had departed from Saul

1 Samuel 17:55-18:16

As Saul watched David going out to meet the Philistine, he said to Abner, commander of the army, “Abner, whose son is that young man? Abner replied, “As surely as you live, Your Majesty, I don’t know. Whose son are you, young man?” Saul asked him. David said, “I am the son of your servant Jesse of Bethlehem.
After David had finished talking with Saul, Jonathan became one in spirit with David, and he loved him as himself.  From that day Saul kept David with him and did not let him return home to his family. And Jonathan made a covenant with David because he loved him as himself. Jonathan took off the robe he was wearing and gave it to David, along with his tunic, and even his sword, his bow and his belt.

When the men were returning home after David had killed the Philistine, the women came out from all the towns of Israel to meet King Saul with singing and dancing, with joyful songs and with timbrels and lyres. As they danced, they sang: “Saul has slain his thousands, and David his tens of thousands.”

Saul was afraid of David, because the Lord was with David but had departed from Saul. So he sent David away from him and gave him command over a thousand men, and David led the troops in their campaigns. In everything he did he had great success, because the Lord was with him. When Saul saw how successful he was, he was afraid of him. But all Israel and Judah loved David, because he led them in their campaigns.

Why?

Because the Lord was with David and David everything was successful and won the hearts of Israelites. But Saul was afraid David, would take away his kingdom.

Jonathan loves David and gives David everything he has. They made a covenant to take each other.

Saul sent David away from him and gave him command over a thousand men and led the troops in their campaigns, Saul had this idea to let David be killed in the war, but because of the blessing of the Lord, David did everything had great success.

How?

When the Lord is with us we can do everything smoothly and be blessed and welcome by many people around us.