Q: Why are priests called to practice chastity?
A: Think of it this way: a priest is called to be faithful to his spouse - the Church - the same way that a husband is called to be faithful to his wife and family. A priest takes care of his "family" the same way that a husband takes care of his, except the priest has a spiritual, and not physical, family.
We are children of God made in his image and likeness - we're all called to glorify God with our bodies, which is practicing chastity, but everyone has a specific vocation in which they do this. Priests glorify God by giving their entire selves - body and soul - to the Church.