How suffering in Purgatory is coupled with joy.
Know that what man deems perfection in himself is in God's sight faulty, for all the things a man does which he sees or feels or means or wills or remembers to have a perfect seeming are wholly fouled and sullied unless he acknowledge them to be from God. If a work is to be perfect it must be wrought in us but not chiefly by us, for God's works must be done in Him and not wrought chiefly by man.
Such works are those last wrought in us by God of His pure and clean love, by Him alone without merit of ours, and so penetrating are they and such fire do they kindle in the soul, that the body which wraps it seems to be consumed as in a furnace never to be quenched until death. It is true that love for God which fills the soul to overflowing, gives it, so I see it, a happiness beyond what can be told, but this happiness takes not one pang from the pain of the souls in Purgatory. Rather the love of these souls, finding itself hindered, causes their pain; and the more perfect is the love of which God has made them capable, the greater is their pain.
So that the souls in Purgatory enjoy the greatest happiness and endure the greatest pain; the one does not hinder the other.