John Gill Commentary 1 Samuel 17:42

"And when the Philistine looked about, and saw David, he disdained him; for he was but a youth, and ruddy, and withal of a fair countenance." — 1 Samuel 17:42 (ASV)

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.