As a Calvinist myself, I will explain exactly why I came to believe Calvinism. I have not always been a Calvinist, but came to believe it several years ago.
Before I begin though, I would like to point out a potential misunderstanding. I'm not sure if this is the way the post was originally intended, but it is deductively invalid to make an argument based on the "real reason" that someone believes something - e.g. the "real reason" that people believe in Calvinism. It is a form of a genetic fallacy (which is when the origin of an idea is confused with its justification - problem being that the reason that someone believes something may not be the same as the reason something is true) - more specifically it is a form of the ad hominem class of arguments.
The simplist answer to why I became a Calvinist is that I became a Calvinist because of Scripture. Calvinism is defined essentially by the five following points:
1. Total depravity - men do not have any innate spiritual goodness in and of themselves and are not inclined towards spiritual good. We are completely unable to please God on our own strength. Jesus says in John 6:44, "No one can come to me unless the Father who sent me draws him, and I will raise him up at the last day." Is. 64:6 compares our righteousness to "filthy rags," and Romans 8:7-8 says "...the sinful mind is hostile to God. It does not submit to God's law, nor can it do so. Those controlled by the sinful nature [NASB "in the flesh"] cannot please God." (NIV)
2. Unconditional Election - God has chosen for salvation and has rejected others (see, for example, Romans 9 or Ephesians 1, which make no sense except in light of predestination; these chapters are about as clear an affirmation of predestination as you could ask for).
3. Limited Atonement - Jesus died only for the elect (those who would accept him). The meaning of this point may not immediately be clear, but in any case the easiest way to think of it is that, if God had not predestined any to salvation, Jesus would not have died. In John 10, Jesus states that He will lay down His life for His sheep.
4. Irresistable grace - it is only by the Grace of God that we are able to accept salvation. God's grace is, moreover, irresistable - i.e. those whom God has predestined are unable to choose to resist that.
5. Perseverence of the Saints - someone who has been truly saved cannot "turn away" or lose their salvation. 1 John 2:19 implies that those who do turn away were never really saved to begin with: "They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us." (NIV) Our Lord says in John 10:26-29, "...you do not believe because you are not my sheep. My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one can snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father's hand." (NIV, emphasis mine). Here, Jesus implies several things: first, only His sheep will believe (and, in light of the rest of Scripture, only His sheep are even capable of believing). Second, our Salvation is guaranteed by Jesus Himself.
