Because of a vow made by his mother. When Dona Glória's first son was born dead, she asked God that the second might survive and promised that, if it were a boy, he would become a priest. She told her husband nothing. When she was widowed, she felt the terror of being separated from her son, but she was very devout: she gathered witnesses to the obligation and had the boy learn his first letters, Latin and catechism at home, with Father Cabral, in order to delay the separation as much as possible.
What decides the sending is José Dias's denunciation, in November 1857: upon learning that her fifteen-year-old son is "hanging around in corners" with the neighbor Capitu, Dona Glória decides to fulfill the vow. Bentinho has no vocation and tries to persuade his mother in a private conversation, but she answers that vocation comes with habit and that she will not ask to be released from her vow.
Before leaving, Bentinho and Capitu swear to marry one another, "come what may." Father Cabral, the old Latin teacher of the house, proposes a middle way: two years in the seminary to test the vocation and, if it does not come, another career. Bentinho enters the São José seminary and stays there until he is seventeen, when the vow is settled by a substitute.