First, the Church does not force celibacy on priests. A priest freely chooses celibacy, a choice he makes long before he’s ordained. Priestly celibacy is a charism of the Holy Spirit, a gift, a special grace that depends on a free-will response by the receiver:
[It] is to be welcomed and continually renewed with a free and loving decision as a priceless gift from God, as an "incentive to pastoral charity" as a singular sharing in God’s fatherhood and in the fruitfulness of the Church, and as a witness to the world of the eschatological kingdom. (John Paul II, Pastores Dabo Vobis, 29)
Second, there are no objective studies that link mandatory celibacy to the priest shortage. If that were the case, then how does one explain the recent growth in priestly vocations worldwide? The late Cardinal John J. O’Connor said many of the men and women he talked to about vocations to religious life put the celibacy issue far down on their list of reasons for hesitating or turning away. A far more common reason was that no one ever seriously asked them (see "Celibacy Isn’t the Problem" by Cardinal John O’Connor).