Because the Bible is confirmed to be the very words of God ten times over with many evidences.
You can check to see the many evidences that back up God's Word here:
Love Branch: Evidences for the Word of God
The poster who thought they heard Jesus actually is receiving a message that conflicts with God's Word. They heard what they believed to be God saying, "Jesus loves you" confirming their unbiblical view of Once Saved Always Saved. Yet, nowhere does the Bible teach this doctrine, and nowhere does the Bible say that Jesus unconditionally loves us. Actually, Jesus says, "If ye keep my commandments, ye shall abide in my love; even as I have kept my Father's commandments, and abide in his love." (John 15:10). Granted, I am sure Jesus does love all people to some degree, but it is not the kind of love that will abide in a person and neither save them. A person needs to keep His commandments to abide in His love; Also, 1 Corinthians 16:22 says that "if any man loves not the Lord Jesus Christ, let him be accursed."