Why does St. Paul say that love (charity) is the greatest?
The Holy Apostle teaches this truth, because when we get to heaven, faith and hope will cease. Since faith is belief and trust in the unseen, faith will no longer be necessary, because in heaven we will see and possess God! Hope ends, because by being in heaven we can no longer desire what we already possess.
But for all eternity we shall have charity: we can love God forever!
Charity is the theological virtue by which we love God above all things for His own sake, and our neighbor as ourselves for the love of God” (CCC, Glossary, p. 870; cf. 1822). Charity, along with Faith and Hope, are called theological virtues because each of them relates us to
God in a personal relationship. “Charity” is about loving God before all things of the earth – thus when we part from our material goods – we are offering them up in communion with God and Christ’s Church.