It's a very good question. In this post, we'll be investigating the whys behind the Real Presence through three main topics: Jesus' teaching as recorded in Scripture, Jesus' teaching as preserved through Sacred Tradition, and, as a plus, Jesus' teaching confirmed through Eucharistic miracles.
In Chapter Six of John's Gospel, Jesus teaches very clearly on the Real Presence: "I am the bread of life. Your ancestors ate the manna in the wilderness, yet they died. But here is the bread that comes down from heaven, which anyone may eat and not die. I am the living bread that came down from heaven. Whoever eats this bread will live forever. This bread is my flesh, which I will give for the life of the world." (John 6:48-51, NIV)
And while Jesus often speaks in metaphors and parables, the reaction of the crowd is not a reaction to a metaphor or parable, but to a literal fact: "How can this man give us his flesh to eat?" (John 6:52)
If the crowd is misunderstanding Jesus, He should explain to them that they have the wrong idea. But He doesn't. Instead, He repeats His words with even more emphasis on their literal meaning: “Very truly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise them up at the last day. For my flesh is real food and my blood is real drink." (John 6:53-55)
This teaching is too much for many of the disciples: "This is a hard teaching. Who can accept it?" (John 6:60) Indeed, many of them turn away entirely from Jesus and leave. "From this time many of his disciples turned back and no longer followed him." (John 6:66)
Jesus did not call back these disciples who left because of a lack of faith. He did not tell them, "Wait, you're misunderstanding me, you have it all wrong." Instead, He turned to His Apostles and said, "Will you, too, go away?"
Interestingly, a note on my Bible points out that the word John uses for "eat" is not the normal verb used to describe human consumption of food, but the verb describing animal eating - the equivalent of "chew" or "gnaw." This may be yet another indicator that Jesus is speaking literally in John 6.
Early Christians revered and appreciated the Real Presence, as shown in these quotes from some of the Church Fathers:
"I have no taste for corruptible food nor for the pleasures of this life. I desire the Bread of God, which is the flesh of Jesus Christ, who was of the seed of David; and for drink I desire His blood, which is love incorruptible." - St. Ignatius of Antioch, c. 110 A.D.
"For not as common bread nor common drink do we receive these; but since Jesus Christ our Savior was made incarnate by the word of God and had both flesh and blood for our salvation, so too, as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by Him, and by the change of which our blood and flesh is nourished, is both the flesh and the blood of that incarnated Jesus." - St. Justin the Martyr, c. 100-165 A.D.
"So long as the prayers of supplication and entreaties have not been made, there is only bread and wine. But after the great and wonderful prayers have been completed, then the bread is become the Body, and the wine the Blood, of our Lord Jesus Christ." - St. Athanasius, c. 295-373 A.D.
It is not only Church Fathers and more modern Catholics who take Jesus' words literally. Martin Luther believed in the Real Presence of Christ in the Eucharist:
"Who, but the devil, has granted such license of wresting the words of the holy Scripture? Who ever read in the Scriptures, that my body is the same as the sign of my body? or, that is is the same as it signifies? What language in the world ever spoke so? It is only then the devil, that imposes upon us by these fanatical men. Not one of the Fathers of the Church, though so numerous, ever spoke as the Sacramentarians: not one of them ever said, It is only bread and wine; or, the body and blood of Christ is not there present.
Surely, it is not credible, nor possible, since they often speak, and repeat their sentiments, that they should never (if they thought so) not so much as once, say, or let slip these words: It is bread only; or the body of Christ is not there, especially it being of great importance, that men should not be deceived. Certainly, in so many Fathers, and in so many writings, the negative might at least be found in one of them, had they thought the body and blood of Christ were not really present: but they are all of them unanimous.”
If Jesus' words and constant tradition are not enough, there have been many Eucharistic miracles throughout the centuries which point to the Real Presence of Christ in the Eucharist.
Countless times, consecrated hosts have bled when desecrated.
The Blessed Sacrament has transformed into visible flesh and blood before the eyes of the doubting.
Animals have bowed before the Eucharist.
Again and again, God has shown us, "This is my beloved Son, with whom I am well pleased." He has given us spectacular signs to help our wavering faith. He has revealed Himself to us poor doubting Thomases.
It's getting late and I'm getting lazy, so instead of going into detail on this topic, I'm going to cheat and give you a book recommendation. :)
In her book Eucharistic Miracles, Joan Carroll Cruz explores many of these amazing instances in great detail. The stories in this book are absolutely astounding. Not only are the events recorded remarkable - they're believable, backed up by documentation, eyewitness accounts, and even scientific experimentation. I highly recommend this book to anyone interested in Eucharistic miracles.
Catholics believe that Jesus' presence in the Eucharist is literal because that's what Jesus taught. This doctrine is as ancient as the Church itself, backed up not only by Scripture but by Church Fathers from the second century. On top of all this, the Real Presence has been manifested in many historical Eucharistic miracles.
Hopefully this post has been helpful in some way or another! Now, my dear brethren in Christ, shall we talk? Have you any questions? Comments? Concerns? Any suggestions for the betterment of this feature? Any suggestions for upcoming Catholicism Explained posts?