51 Thus the Catholic priest is freed from the bonds of a family and of self-interest, - the chief bonds which could bind him too closely to earth. Thus freed, his heart will more readily take flame from that heavenly fire that burns in the Heart of Jesus; that fire that seeks only to inflame apostolic hearts and through them "cast fire on all the earth." This is the fire of zeal. Like the zeal of Jesus described in Holy Scripture, the zeal of the priest for the glory of God and the salvation of souls sought to consume him. It should make him forget himself and all earthly things. It should powerfully urge him to dedicate himself utterly to his sublime work, and to search out means ever more effective for an apostolate ever wider and ever better.
Source: Ad Catholici Sacerdotii (Vatican.va)