John Gill Commentary 1 Samuel 18:20

"And Michal, Saul`s daughter, loved David: and they told Saul, and the thing pleased him." — 1 Samuel 18:20 (ASV)

And Michal, Saul's daughter, loved David
His youngest daughter fell in love with him, because of the comeliness of his person, his gallant behaviour, his wise conduct, and the general esteem and reputation he was had in, as may be supposed:

and they told Saul, and the thing pleased him ;
not that his daughter loved David, or that he should be his son-in-law, but that he should have an opportunity, as he hoped, of destroying David, which he had lost by giving his elder daughter to another; as also of retrieving his credit with the people, which was greatly sunk by using David in the manner he did, who had become the darling of the people.