In today's Daily Office readings, there is following the curious verse:
"Now I rejoice in what was suffered for you, and I fill up in my flesh what is still lacking in regard to Christ's afflictions, for the sake of his body, which is the church."
It seems so out of place coming from St Paul who lays great stress on the supreme sufficiency of the atoning suffering and death of Jesus Christ, i.e., that nothing is lacking. The Amplified Bible adds a little more softening detail: "In my own person I am making up whatever is still lacking and remains to be completed [on our part] of Christ's afflictions." In contrast, the Rheims New Testament puts it a little more abrasively, where Paul says, "I fill up those things that are wanting of the sufferings of Christ."
I think some depth of understanding can be found in the New Jerusalem Bible, which reads, "It makes me happy to be suffering for you now, and in my own body to make up all the hardships that still have to be undergone by Christ for the sake of his body, the Church." The note on this verse explains, "Jesus suffered in order to establish the reign of God, and anyone who continues his work must share this suffering. Paul is not claiming to ass anything to the redemptive value of the cross (to which nothing is lacking); but he associates himself with the trials of Jesus, by his suffering in his apostolate, see 2 Co 1:5; Ph 1:20i. These are the sufferings predicted for the messianic era, Mt 24:8; Ac 14:22m; 1 Tm 4:1a, and are all part of the way in which God had always intended the Church to develop; Paul feels that, being the messenger Christ has chosen to send to the gentiles, he has been especially called to complete these sufferings."
What is "lacking" in the sufferings of Christ? It is not his sufferings, but our share in them. I would say that the main point is that since Christ suffered, all suffering has a special dignity and redemptive meaning. Thus, his own sufferings being Paul joy because he feels that his sufferings draw him closer to Christ. We know from mere human experience that those who share some affliction often share a special communion. I'll let Pope John Paul II have the last comment on that, since he talked about it so movingly in his encyclical The Gospel of Life:
"Living to the Lord also means recognizing that suffering, while still an evil and a trial in itself, can always become a source of good. It becomes such if it is experienced for love and with love through sharing, by God's gracious gift and one's own personal and free choice, in the suffering of Christ Crucified. In this way, the person who lives his suffering in the Lord grows more fully conformed to him (cf. Phil 3:10; 1 Pet 2:21) and more closely associated with his redemptive work on behalf of the Church and humanity. This was the experience of Saint Paul, which every person who suffers is called to relive: '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'" (Section 67).