Why do we evangelize (and Calvinists are the leaders in this field!)? Because of what the Bible says.
Why do we believe in the unconditional election by God of some to salvation? Because this is what the Bible says.
I notice you quoted John 3:16,17. Good. Now, go look at those verses within the context of Jesus' conversation with Nicodemus. Do you notice that Jesus is talking about the need for a "new birth"? Do you realize that "new birth" is from God? We don't just wake up one morning and say "I think I will be born again today."