The answer to this question has to do with the curious and compelling fascination that the bear manages to exert on the boy and on all of the hunters who come year in and year out to try and "hunt" the bear, though of course, during the course of the story, the boy discovers that they have no serious intention of killing the bear. It is the boy's first failure to see the bear whilst it is close to him that births the desire and determination within him to see it. Note what he says, justifying his response:
So I must see him, he thought. I must look at him. Otherwise, it seemed to him that it would go on like this forever, as it had gone on with his father and Major de Spain, who was older than his father, and even with old General Compson, who had been old enough to be a brigade commander in 1865. Otherwise, it would go on so forever, next time and next time, after and after and after. It seemed to him that he could never see the two of them, himself and the bear, shadowy in the limbo from which time emerged, becoming time; the old bear absolved of mortality and himself partaking, sharing a little of it, enough of it.
The boy therefore feels the need to objectively look at the bear in fact rather than the myth he has become. It appears to him that there is a cycle going on through the generations, of an inability to see the bear, that he wants to break. Only then can he begin to penetrate the mystery of the bear and understand the magnetic compulsion that it yields on both him, his father and others.