To understand the Bread of Life Discourse we must recognize the immediate context of this sermon which Jesus gave and John records in the sixth chapter of his Gospel. The Bread of Life Discourse is in parallel with the multiplication of the loaves and fish, with Jesus walking on the stormy waters to rescue the disciples being in the middle and at the heart of the two.
Jesus comes to the disciples upon the water, but they cannot recognize him with their sight, it is only when they hear his voice "It is I, do not be afraid" that they have faith and receiving him into the boat to save them. This is the key to interpreting the Bread of Life Discourse -- the boat is the Church, the sea is the world, and we are the disciples. We sometimes seem to be all alone in the world, but Jesus comes to us in a manner unrecognizable to our sight (he looks like bread), but we hear his voice "This is my Body, this is my Blood" and we believe and truly receive him into our own bodies and souls.
This is why the Bread of Life Discourse can't be a metaphor and the Eucharist can't be a symbol -- because a metaphor wouldn't have saved the apostles, and they didn't receive a mere symbol of Jesus into the boat.
Finally, the fact that Jesus cares for every little fragment of the multiplied loaves, shows us the reverence we must have for every piece and crumb of the Blessed Sacrament.
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