Why did Paul say 'Eye hath not seen, nor ear heard'?
Paul said it to convey that the things God has prepared for those who love Him—eternal life in His presence forever—are beyond human sight and hearing. After being given a glimpse of that glory, he returned and testified that the things awaiting believers exceed anything perceived now.
Les teaches that the believer’s hope is not merely a better life during the roughly 50 years we have in this world, but eternal life—forever and ever in His presence. He notes that too many people lose sight of that. Paul, in 2 Corinthians, had just a glimpse of it, and then he came back and said, “Eye hath not seen, nor ear heard the things that have been prepared for them that love him.” So the statement is tied to the prepared glory and eternal presence of God that awaits those who love Him, which is beyond what human sight and hearing have grasped.