In the first chapter of Colossians, Paul writes, Now I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ's afflictions for the sake of his body, that is, the church, of which I became a minister according to the divine office which was given to me for you, to make the word of God fully known, the mystery hidden for ages and generations but now made manifest to his saints. Colossians 1:24-26.

We know that there is nothing lacking in Christ's suffering, so we understand that Paul meant something else - that God has chosen to make our sufferings to be associated with Christ.

Saint Paul, like the Mystical Saints, was being mystical. Essentially, he said that if the way he was handling his own suffering pleases God, then he hopes he will bless others as a result.