Why do Catholic's think Communion is the real presence of Jesus?

Q: Doesn’t the Bible say that Jesus is a door and a vine? So how is it different when He says that He is the “Bread of Life”? Was Jesus speaking literally when he said “This is my Body”?
The Bible does say Jesus is the door and the vine, so, the question is:
“How can we be sure that when Jesus said, ‘I am the Bread of Life’ that He was talking about His Real and actual Presence in the Eucharist and not just a metaphor like when He said ‘I am the door’ (John 10:9) or ‘I am the vine’ (John 15:5)?”

Let’s tackle this from a couple of different angles, shall we?
1. We can tell that this is NOT symbolism (like the “door” and “vine” passages that we discussed) because Christ explains what He is saying in even greater, more graphic detail, saying to us “For my flesh is food indeed, and my blood is drink indeed (Jn 6:55).”
2. It’s true that while Jesus used figures of speech and metaphors to explain the Heavenly mystery to folks, there is a difference in context when He says “I am the door” or “I am the vine”, and when He said “I AM the Bread of Life.”
Just look at what happened when He said it…people were confused (Jn 6:42), people grew angry (Jn 6:52)…people even began to walk away (Jn 6:66).
The fact is, however, that He didn’t correct what He said…He said it even louder, and said it again and again and again:
Why didn’t Jesus correct their misunderstanding?
“Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink.” – John 6:53-55

3. Ask yourself, why didn’t Jesus correct their misunderstanding? He was ministering, He wanted people to hear God’s truth and He was building a large following. He would have to be crazy to let them go simply because they ‘misunderstood’ His metaphor. In other cases, when people were confused, Jesus was sure to “clarify” what He was saying (Matthew 16:5-12, for instance). Also, He would have a moral obligation to explain what He “really” meant to them…their salvation was at stake, that’s what He was talking about!

4. Since we weren’t present when He spoke these words, we have to rely on the Scriptures. One of the beautiful gifts of being Catholic is that we hold the Sacred Scriptures and Sacred Tradition in equal esteem, very different but both very important.
Why is that such a gift? Because sentences mean different things when emphasis is placed on different words.
Let me borrow an example from a great Catholic apologist Patrick Madrid…
Read the following sentence:
“I never said that you stole the money.”
Simple sentence, right? The thing is that the meaning can change dramatically, based upon how it is read, and where the emphasis is placed.
Below are six different ways to read that sentence…watch what happens when you change the emphasis:
I never said that you stole the money.”
“I never said that you stole the money.”
“I never said that you stole the money.”
“I never said that you stole the money.”
“I never said that you stole the money.”
“I never said that you stole the money.”
So, what is the right way to read it? Well, if the one who said it isn’t right next to you, you turn to those who were there. Listen to what the witnesses say about how it was said (Tradition), and how they responded to it (traditions).

5. St. Paul wasn’t there that day…he wasn’t even at the Last Supper, but it is astoundingly clear in his letter to the Corinthians, that he believes and proclaims the Real Presence in the Eucharist:

“The cup of blessing that we bless, is it not a participation in the blood of Christ? The bread that we break, is it not a participation in the body of Christ?”
 – 1 Corinthians 10:16
“Therefore whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord. For anyone who eats and drinks without discerning the body, eats and drinks judgment on himself.”
– 1 Corinthians 11:27,29

Why would St. Paul say that? That’s right…Tradition.
Through the handing on of sacred Tradition and studying the works of our early Church Fathers, including the epistles of St. Paul, we come to find out WHERE the emphasis in certain scriptures must have been placed by Christ, Himself…there were eyewitness groups of His followers doing as they had seen and been taught, by God (Christ) Himself. That’s what Sacred Tradition is: the handing on of the faith and the practices of faith, instituted by Christ.