Here's my rationale, friend. Hope this helps.
Many times in the old testament the jews were commanded to eat of the sacrifice that was offered to God on their behalf. Likewise, Christ commands us to eat of the once and for all eternal sacrifice that He offered on behalf of us, as our ultimate high priest.
I pulled the following exerpt describing OT practices from "Christ in the Old Testament" (http://www.alccok.com/ChristintheOTsacrifices.htm
"The peace offering consisted of a sacrificed animal, some which was burned to the Lord, some eaten by the priest, and some returned to the worshiper to eat. "This sacrificial act was always a social occasion, the worshipper invited friends to the meal, to eat and to drink before Jahweh. This sacrifice which, more than any other, came into the category of a communion sacrifice, the participants knew Jahweh to be invisibly present as the guest of honor."
Can you see the parallel between the old and the new covenant here? It was after I noticed this that I realized what Christ truly meant when he said to eat of his body, and why it was so important for us to do so. This is because His divine flesh is the eternal sacrifice by which our bodies are to be nourished and incorporated into the mystical and physical Body of Christ. This is the actual physical means by which we partake of His sacrifice and become united with Him unto life eternal!
From John 6: (I pulled this verse from a hyper-literal translation because I think it is important here)
"The one eating My flesh and drinking My blood remains in me, and I in him. Just as the living Father sent me, and I live because of the Father, also the one feeding on Me, that one will also live because of Me. . . But there are some of you who do not believe"
Two active key requirements I draw from this passage: believe, and eat.
Later, at the Last supper, Christ makes it abundantly clear to his disciples what he meant in the above passage.
The letters of Paul clearly view it as partaking of his body and of his blood: (also hyper-literal)
1 Cor. 10:15-18
I am speaking as to wise persons; you judge what I am saying. The cup of the blessing which we bless, it is the fellowship of (or, a sharing in) the blood of Christ, is it not? The bread which we break, it is the fellowship of the body of Christ, is it not? Because we, the many are one bread, one body, for we all partake of the one bread. Be watching Israel according to the flesh. The ones eating the sacrifices are participants in the altar, are they not?
As do all the writings of the early church fathers regarding the matter, especially Ignatius of Antioch, who in particular, being very influential in my mind:
Ignatius of Antioch, Epistle to the Smyrnaeans, Chapter 6, 110 A.D.:
Take note of those who hold heterodox opinions on the grace of Jesus Christ which has come to us, and see how contrary their opinions are to the mind of God ... They abstain from the Eucharist and from prayer because they do not confess that the Eucharist is the flesh of our Savior Jesus Christ, flesh which suffered for our sins and which that Father, in his goodness, raised up again. They who deny the gift of God are perishing in their disputes.
St. Ignatius of Antioch, Epistle to the Smyrnaeans, 8:1, 110 A.D.:
Let that Eucharist be held valid which is offered by the bishop or by the one to whom the bishop has committed this charge. Wherever the bishop appears, there let the people be; as wherever Jesus Christ is, there is the Catholic Church.
St. Ignatius of Antioch, Epistle to the Romans, 7, 110 A.D.:
I desire the Bread of God, the heavenly Bread, the Bread of Life, which is the flesh of Jesus Christ, the Son of God, who became afterwards of the seed of David and Abraham; I wish the drink of God, namely His blood, which is incorruptible love and eternal life.
St. Ignatius of Antioch, Epistle to the Philadephians, 4:1, 110 A.D.:
Be ye careful therefore to observe one eucharist (for there is one flesh of our Lord Jesus Christ and one cup unto union in His blood; there is one altar, as there is one bishop, together with the presbytery and the deacons my fellow-servants), that whatsoever ye do, ye may do it after God.
So it was simple: I had to find a church that believed and practiced the Lords supper like the early Church did. The relationship between theosis and the eucharist was a amazing revelation for me once I started to grasp it.