why did buck attack Spitz on the shore of Lake Le Barge?

"Close in under the sheltering rock Buck made his nest. So snug and warm was it, that he was loath to leave it when Francois distributed the fish which he had first thawed over the fire. But when Buck finished his ration and returned, he found his nest occupied. A warning snarl told him that the trespasser was Spitz. Till now Buck had avoided trouble with his enemy, but this was too much. The beast in him roared. He sprang upon Spitz with a fury which surprised them both, and Spitz particularly, for his whole experience with Buck had gone to teach him that his rival was an unusually timid dog, who managed to hold his own only because of his great weight and size."