Why do the faithful have to say "And with your spirit " in the Catholic Mass?

In the Catholic Holy Mass (Ordinary Form of the Mass in the Latin rite), the prayer for peace is said more than once . But, when the priest says "Peace be with you"the faithful respond by saying "And with your spirit ", rather than saying "And with you". I wish to know why the faithful are required to pray for peace `with the spirit of the celebrant'.