Pope Paul VI
Sacerdotalis Caelibatus §65
Sacerdotalis Caelibatus: On the Celibacy of the Priest
65 After the capability of a man has been ascertained and he has been admitted to the course of studies leading to the goal of the priesthood, care should be taken for the progressive development of a mature personality through physical, intellectual and moral education directed toward the control and personal dominion of his temperament, sentiments and passions.
Source: Sacerdotalis Caelibatus (Vatican.va)