Suppose that you have been away from Church for awhile, physically or perhaps also morally. For whatever reason, the time seems right and you decide to come return some Sunday morning.
Maybe people know you and will be glad to see you again or you'll be recognized as someone new and greeted well. You get lots of handshakes, perhaps a few hugs and plenty of "Welcome back"s or "Glad you're here!"s. You're thinking"Hey, this feels pretty good. I do like it here. I will come back again!"
This is wonderful but there is something missing. You're not fully re-united with the community yet, in the most important way: You have yet to be reunited with the spiritual Body of Christ. You've reconnected socially with some of the members of the Body but, if Christ is the shepherd, you're still standing outside the sheepfold. Christ has left the 99 to come look for you, the one, but you haven't yet responded fully to His call of repentance (to which he calls all of us). You're invited to reconciliation with God himself, as in the Sacrament of Penance.
"Those who approach the sacrament of Penance obtain
pardon from God's mercy for the offense committed against him, and are,
at the same time, reconciled with the Church which they have wounded by
their sins and which by charity, by example, and by prayer labors for
their conversion." - CCC 1422
It sounds demanding and exclusionary but it really is awesome and beautiful. Of course it's great to have people in the Church welcome you with friendly faces and outstretched arms but it's even better to have God welcome you with His divine face and arms stretched out on the cross. This is exactly what he does.
In the Sacrament of Penance we accept God's love for us in a real and effective way. Our soul is made clean and we are reunited with God (and also reunited with the people of God more intimately than we ever could be with just human interaction.) The Sacrament is not a barrier to communion but rather a means to enter into deeper communion. It is only a barrier to those who refuse to take the offer of mercy.
Maybe we haven't been away from the Church in the usual sense but since sin separates us from God, we are all continually in need of re-entering communion with Christ and His Church and therefore in need of confessing our sins and answering again God's call to be with Him. Let us take the time to avail ourselves of the wonderful gift of the reconciliation offered in the Sacrament of Penance. (Especially in Advent, as we prepare for Christmas) We will no doubt sin again later but God knows this and loves us just the same. He is ready to forgive as many times as we ask! We long for union with others, and rightly so, but let us be primarily concerned with union with God. If we are united with Him all else will take care of itself.
A Guide to Confession: http://www.catholic.org/prayers/confession.php