When Buck would bite down on John Thornton's hand and leave teeth impressions; what was Buck displaying?

Buck was displaying love and affection.

Buck had a trick of love expression that was akin to hurt. He would often seize Thornton’s hand in his mouth and close so fiercely that the flesh bore the impress of his teeth for some time afterward. And as Buck understood the oaths to be love words, so the man understood this feigned bite for a caress.