Pope Paul VI
Sacerdotalis Caelibatus §67
Sacerdotalis Caelibatus: On the Celibacy of the Priest
67 The educator should skillfully stimulate the young man to the evangelical virtue of sincerity (117) and to spontaneity by approving every good personal initiative, so that the young man will come to know and properly evaluate himself, wisely assume his own responsibilities, and train himself to that self-control which is of such importance in priestly education.
Source: Sacerdotalis Caelibatus (Vatican.va)