This day will Yahweh deliver you into my hand; and I will strike you, and take your head from off you; and I will give the dead bodies of the host of the Philistines this day to the birds of the sky, and to the wild animals of the earth; that all the earth may know that there is a God in Israel,
It was a trophy that David had, and he wasn't gonna let the thing go. He was gonna carry it around for a few days. Must have been a big head too, you know, the guy's nine foot, six inches tall.