Is it to carry on the traditions of your family, attending the same church that Mom & Dad went to, getting married in the same church Mom & Dad got married in, getting your kids baptized in the same church you were baptized in, so that you can be buried in the same cemetery Granny & Gramps are buried in?
Is it a tribal designation, like the old joke: A man got lost in Belfast and wasn't sure if he was on the Catholic or Protestant side of town. Some rough-looking youths came up to him and asked, "Are you a Protestant or a Catholic?" Knowing the wrong answer could get him killed, he answered, "Actually, I'm an atheist." The youths looked puzzled and asked, "Yes, but are you a Protestant atheist or a Catholic atheist?"
Is it to have a sense of belonging and community? Perhaps that which you belong to is less important to you than that you belong, and it might as well be the local country club as the local parish? Or maybe you're simply trying to get the parishioner discount at the parochial school? Maybe using the Knights of Columbus for a little business networking?
These are insufficient reasons. The only reason to be a Catholic, the whole point to it, is that the Catholic faith reveals to us the purpose of life and helps us to fulfill it. I turn now to the first few entries in the venerable Baltimore Catechism.
1. Who made us?
God made us.
2. Who is God?
God is the Supreme Being, infinitely perfect, who made all things and keeps them in existence.
3. Why did God make us?
God made us to show forth His goodness and to share with us His everlasting happiness in heaven.
4. What must we do to gain the happiness of heaven?
To gain the happiness of heaven we must know, love, and serve God in this world.
5. From whom do we learn to know, love, and serve God?
We learn to know, love, and serve God from Jesus Christ, the Son of God, who teaches us through the Catholic Church.
Every single human being who has ever lived has wondered, "What am I doing here?" asking both their origin and their purpose. Origin and purpose are fundamentally bound to one another: everything that is made is made for some reason, for some end, and is given that end or purpose or reason by its maker. Every person knows they have a purpose: they are enlivened when they have one and depressed when they have none. Modern folk tend to think that human beings should determine or create their own ultimate purpose for themselves, but this would only make sense if we were our own creators, which we are not. We did not bring ourselves into existence, so we cannot set our own ultimate reason for existing. That reason is inscribed in our very form, hardwired into us, part of the factory settings, so to speak, unalterable and irrevocable.
Our purpose can only be made known to us by knowing the mind of the one who made us--that is, God. But who is this God who made us? He is the source of all existence, the creator of everything that is, having all perfections. And He made us out of pure generosity, absolute gratuity; He had no need to create us or anything--nothing could compel Him. Simply out of His goodness and His desire to share of Himself, God made us, destined for eternal happiness with Him. All we need do is follow His design for us, His design within us; for since He made us for Himself, our happiness will be in knowing and loving and serving Him. And yet we failed and continue to fail to heed this call, mysteriously rejecting that which will bring us fulfillment. So God comes to our aid, and helps us to know Him and love Him by revealing Himself to us, preeminently in the greatest event in history, in which God Himself condescended to become one of us in the person of Jesus Christ, teaching, dying, and rising, defeating death that we might live. In Christ our sins are forgiven and our unity with God is restored. In Christ we share in the very life of God Himself! And Christ continues his presence and his work on earth through his Body, the Church, built upon the rock of St. Peter, founded on the twelve stones of the Apostles, spread through the preaching of the Gospel message of salvation through Christ, nourished by those visible signs of his invisible grace, the sacraments.
This is the only reason to be a Catholic: to fulfill our destiny by knowing, loving, and serving God, taught by Christ and his Church. Family tradition and identity and belonging will follow from that, but those are ancillary concerns, attendant benefits of the grace of communion with the Triune God.
If this is not your reason for belonging to the Church, for attending Mass, I say: repent and be converted! Make Christ the center of your life! I say this as much to myself as anyone. We all need ever-deeper conversion to Christ, ever-strengthened unity with him, ever-greater love for him. Come and find your fulfillment! Come and find your purpose! Come and find your joy!