Protestant communion is a subjective experience. The bread and wine only symbolize the body and blood of Christ. The communion will affect the person to the degree that he is devoted to the Person of Jesus. If the communicant is devoted to Jesus, the experience becomes an act of devotion and can even be an act of the person’s love. For those not devoted to him, the experience will mean little. For those who receive the Eucharist in the Catholic and Orthodox Churches, the experience is more than merely subjective. The eucharistic elements (over and above their symbolic value) have an objective reality apart from the recipient’s religiosity. These recipients have the inexpressible experience of the intimate visitation of Jesus within their very bodies. Nothing on the face of the earth can equal this.
Olivier did a great job of answering, but I wanted to add:
Because Jesus said so! :) He instituted the priesthood and the sacrament of confession.
Further recommended reading: