and David arose and went, he and his men, and killed of the Philistines two hundred men; and David brought their foreskins, and they gave them in full number to the king, that he might be the king's son-in-law. Saul gave him Michal his daughter as wife.
He set up a dowry regarding the Philistines. David went out and gave him double dowry. So Saul then of course was sort of shocked and surprised. He figured David would get wiped out in going out against the Philistines.