Within an atheistic worldview, suffering is meaningless and regrettable. Of course, without a Creator, everything in the universe is, in a certain sense, meaningless sound and fury, signifying nothing. Non-Christian religions get a step closer, but still view suffering as something to avoid, escape or deny. To fully understand suffering requires the Christian revelation, and specifically, the Cross and the Church.
The Cross and Suffering
So all Christian groups see a connection between the Cross and suffering. On the precise relationship between the two,Catholics and Protestants tend to part company (a fact that emerges most clearly in debates surrounding Purgatory). Protestants tend to focus on Christ’s suffering on the Cross as a substitution: He went on the Cross in our stead. That’s true, but it’s not the full story. Christ doesn’t just go to the Cross in our place: He calls us to join Him there. That’s the meaning behind Christ’s radical call to His would-be disciples: “If any man will come after Me, let him deny himself, and take up his cross, and follow Me” (Matthew 16:24)
I suspect that the difference in the way that Catholics and Protestants view the Cross and suffering comes from a difference in how we understand Christ’s relation to the Church. Protestantism tends to focus on the radical otherness of Christ: the distance between our fallen nature and His Nature is infinite. Again, this is true, but not the full story. Yes, the gap between us and God is infinitely wide. But God, being infinite, is capable of bridging it, and has. Christ has done two radical things. First, He takes on our humanity in the Incarnation. Second, He invites us to share in His Divinity, as various passages in Scripture (like 2 Peter 1:4 and 1 John 3:2) make clear. So there is an infinite gap by nature, but Christ bridges that gap.
The specific way that Christ invites us to share in His Divinity is through His Church. Baptized Christians are incorporated into His Mystical Body (Romans 12:5; 1 Corinthians 12:27). So the “Whole Christ” (Christus totus) is Jesus, the Head (and the “Unique Christ”) in union with His Body, the Church (CCC 795; CCC 793). As St. Paul says in Ephesians 1:22-23, “God placed all things under His feet and appointed Him to be Head over everything for the Church, which is His Body, the fullness of Him who fills everything in every way.” That’s a radical claim: that Jesus Christ plus the Church is “the fullness of” Christ. But there it is, from St. Paul himself. It is for this reason that what Jesus begins in His Person, He continues in the Church. In a sense, the Church continues the Mystery of the Incarnation throughout history.
The Church as a Continuation of the Life, Death, and Resurrection of Christ
These passages are fulfilled, in a sense, when the Magi bring gifts to Christ on Epiphany. But this fulfillment is just the beginning. The Magi come “from the East” (Matthew 2:1), while the Old Testament prophesies include gifts being brought from all over the place. Sheba and Midian are south of Israel, while Tarshish (believed to be either Carthage or part of modern-day Spain) brings gifts by ship (Isaiah 60:9), almost certainly from the Mediterranean to the west.
So the Magi begin to fulfill the prophesy, but they don’t full it completely. The Epiphany is begun with the Magi from the east, but won’t truly end until the Church succeeds in Her Commission to “go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit” (Matthew 28:20) and all “Nations will come to your light, and kings to the brightness of your dawn,” (Isaiah 60:3).
The Church and Suffering
It's in this sense that we understand St. Paul's bold statement in Colossians 1:24, “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.” In His Flesh, Christ's suffering is finished. In His Mystical Body, the suffering continues, and unites us to Jesus in His Passion.
In this way, we become more like Christ (Philippians 3:21). And as Paul says in Romans 8:17, this is the only road to the Resurrection: “if we are children, then we are heirs—heirs of God and co-heirs with Christ, if indeed we share in His sufferings in order that we may also share in His glory.”
You can't always be “happy” while suffering, but you should strive to have joy, that you've been found worthy to carry within yourself the Crucified Christ, to manifest His beautiful Passion to the world, in a way uniquely reserved for you, from all eternity.