Saul commanded his servants, [saying], Commune with David secretly, and say, Behold, the king has delight in you, and all his servants love you: now therefore be the king's son-in-law.
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.