He is considered a stranger because he is physically different from the other members of the caste he belongs to.
Explanation:
The character Bernard Marx feels dissatisfied with the world where he lives, in part because he is physically different from the members of his caste, because of the process of incubation that gave birth to him, where alcohol was placed in the place of blood and this compromised the growth. In a redoubt where people live in the mold of the past a kind of "historical reserve" - similar to the current Indian reservations - where the "wild" customs of the past are preserved, Bernard meets a woman from the civilization, Linda, and her son , John. Bernard sees a possibility of gaining social respect by presenting John as a copy of the savages to civilized society.
