From Catholic365.com:
“What is the wafer you all are receiving, and can I get one, too?
The Source and Summit of everything we do as Catholics is the Eucharist, aka: Holy Communion. Jesus said in the Gospel of John 6:51, “I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world.” We take this very literally. At the Last Supper Jesus took bread and wine, blessed them and said, “This is my body” “This is my blood” and “Do this in memory of me.” (Luke 22) We take this, with the Gospel of John very literally, too. Jesus’ Sacrifice for us on the Cross is what we celebrate at Mass. He made that Sacrifice once and for all, but told us to participate in it, too. Our celebration of this Sacrifice isn’t a new one; it brings the one and only Sacrifice into the present. The hosts and wine change from those elements to His True Presence—to the Body and Blood of Christ. It’s not a symbol for us. It’s a reality. And so, we ask that only those who are united with us in that belief through the Catholic Church participate in receiving Communion.”
And as someone says later in this thread, its class or I would call it politeness. I’m not going to go into an Orthodox Temple and insist on sitting with the men or hug my Jewish neighbor on the Sabbath. Do I believe that I would make him unclean? No but he does and so I respect his faith.
When I was 13 years old I went through confirmation at the Episcopal church in which I was raised. I had a hunger for God that my parents, who did not attend church, were not particularly interested in fulfilling. Friends of theirs who were Catholic took me with them to Mass on Sundays every now and then, which I enjoyed...however, not having any idea about the True Presence, I would go up and receive Communion! I don’t know why on Earth they let me! I just thought “hey I’ve been confirmed so I can do this now, isn’t that wonderful!” But, wouldn’t you know that around the age of 35 I converted and am now Catholic. Guess the Lord does work in mysterious ways!
As to your last point, you’re not Jewish so, of course, you would not presume.
As to the rest I have a history that makes me unable to accept Christians judging one another.