When buck is running with the wild brother why does he stop and return to Camp chapter 7

He returns to the valley to remember John Thorton.

In the summers there is one visitor, however, to that valley, of which the Yeehats do not know. It is a great, gloriously coated wolf, like, and yet unlike, all other wolves. He crosses alone from the smiling timber land and comes down into an open space among the trees. Here a yellow stream flows from rotted moose-hide sacks and sinks into the ground, with long grasses growing through it and vegetable mould overrunning it and hiding its yellow from the sun; and here he muses for a time, howling once, long and mournfully, ere he departs.