Vocations 

Becoming a priest or a man or woman religious is not primarily our own decision....

Rather it is the response to a call and to a call of love.

—Pope Francis, Address to Seminarians and Novices, July 6, 2013

The Sacrament of Holy Orders is the continuation of Christ's priesthood, which He bestowed upon His Apostles; thus, the Catechism of the Catholic Church refers to the Sacrament of Holy Orders as "the sacrament of apostolic ministry."

Because of his role as a successor to the Apostles, who were themselves successors to Christ, the bishop is the proper minister of the sacrament. The grace of sanctifying others that he receives at his own ordination allows him to ordain others.

From: Learn About the History of the Sacrament and the Three Levels of Ordination; By Scott P. Richert, Catholicism Expert