1 Samuel 18:20

And Michal Saul’s daughter loved David: and they told Saul, and the thing pleased him.

Michal, the daughter of Saul, loved David, and when this affection was reported to Saul it pleased him. His pleasure was not fatherly delight but scheming satisfaction, for he saw in her attachment another opening to work against David. He therefore offered Michal to David in marriage on the condition that David first defeat and kill a specified number of Philistines.

The demand was a trap, laid in the hope that David would fall and die in battle with the Philistines. Instead David returned victorious from the assignment and became Saul's son-in-law. The success of the plan's victim enraged Saul, because it only raised David's standing before the people and confirmed to Saul that David would take his place as king.