1 Samuel 17:42

And when the Philistine looked about, and saw David, he
disdained him
He looked about for his antagonist, to take a view of him, what sort of a man he was, expecting to see one much like himself; but observing a puny young man, he despised him in his heart, and perhaps looked upon it as an affront to him to send such a man to fight with him:

for he was [but] a youth;
his age was one reason why he despised him, being, as before observed, about twenty years of age, and not come to his full strength, a stripling, as he is called, ( 1 Samuel 17:56 ) , another reason follows,

and ruddy, and of a fair countenance;
looked effeminate, had not the appearance of a soldier, of a weather beaten veteran, exposed to heat and cold, and inured to hardships.