We are called by Christ to love as he loved, that is, we are called to love perfectly. What happens if we die without being in a state of unrepentant mortal sin, but also without having learned to love perfectly? Will we go to heaven as imperfect creatures that have no knowledge of how to receive and give perfect love? No.
Purgatory is a state where we get purged of our inordinate attachments to creation, and where our love is purified of these inordinate attachments. Once we are purified of our selfish and mercenary love, we can both love perfectly, and receive perfect love. If we haven’t learned how to love God with our whole hearts, our whole minds, and our whole souls while we are on earth, we will learn how to do this in Purgatory.
This world can be our purgatory, if we allow it to be.**Catechism of the Catholic Church
1030 ** All who die in God’s grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation; but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven.
Apart from the happiness of the saints in heaven, I think there is no joy comparable to that of the souls in purgatory. An incessant communication with God renders their happiness daily more intense, and this union with God grows more and more intimate, according as the impediments to that union, which exist in the soul, are consumed. These obstacles . . . are the rust and the remains, as it were, of sin; and the fire continues to consume them, and thus the soul gradually expands under the divine influence. Thus, according as the rust diminishes and the soul is laid bare to the divine rays, happiness is augmented. The one grows and the other wanes until the time of trial is elapsed . . . With regard to the will of these souls, they can never say that these pains are pains, so great is their contentment with the ordinance of God, with which their wills are united in perfect charity.
Purgatory is not, as Tertullian thought, some kind of supra-worldly concentration camp where one is forced to undergo punishments in a more or less arbitrary fashion. Rather it is the inwardly necessary process of transformation in which a person becomes capable of Christ, capable of God * and thus capable of unity with the whole communion of saints. Simply to look at people with any degree of realism at all is to grasp the necessity of such a process. It does not replace grace by works, but allows the former to achieve its full victory precisely as grace. What actually saves is the full assent of faith. But in most of us, that basic option is buried under a great deal of wood, hay and straw. Only with difficulty can it peer out from behind the latticework of an egoism we are powerless to pull down with our own hands. Man is the recipient of the divine mercy, yet this does not exonerate him from the need to be transformed. Encounter with the Lord is this transformation. It is the fire that burns away our dross and re-forms us to be vessels of eternal joy.