Goliath was likely somewhat offended by the Israelites' champion. He saw a man who did not yet have a full beard (ruddy), and thus not someone with a lot of battle experience. To his mind, David had no chance against him,1 and he was an insult to Goliath. Goliath was expecting a more experienced and accomplished fighter.
41 And the Philistine moved forward and came near to David, with his shield-bearer in front of him.