Catholics believe that the Communion we take is truly Jesus Christ, body, blood, soul, and divinity.  We do not consider the Holy Eucharist as a symbol.  It is actually Jesus.  In 1 Corinthians, we read that anyone who eats the bread and drinks the cup of the Lord without proper discernment brings condemnation onto themselves.  (paraphrased from 1 Cor. 21-31).  It is only right for the Catholic church to help a person understand the magnitude of communion prior to allowing one to receive.  Also, when one receives the Eucharist, one is saying he/she is "in communion" with the beliefs of that church.  If you do not follow the believes of the Catholic Church you are not in communion with all those receiving.