Jesus teaches us a very profound truth...in v.24 He said, "Truly, truly, I say to you, unless a grain of wheat falls into the earth and dies, it remains alone; but if it dies, it bears much fruit." Jesus is saying, 'death is the way to life'. Yet so many never understand this teaching and thus seek for life in their short stay on this earth. This is why Jesus teaches in v.25, "He who loves his life loses it...", because as He also taught, 'narrow is the way to life and FEW there be who enter thereby...". The man or woman who loves his earthly life and thus lives to find life in this life...ultimately lose REAL life eternally with God. How sad...but how true this is...look at so many, even nominal Christians, who are living for all the life they can get out of their time on earth. Conversely, Jesus continues in v.25 by teaching that "...he who hates his life in this world will keep it to life eternal." To 'hate his life' means to be so committed to Christ and His gospel that you have no 'self-centeredness’; Christ, His life, His commission, His calling, His gifting, His love, His body, His return...have arrested your self-interest and given all of life NEW meaning and hope and direction. Whatever you do is now for the glory of God and not for self promotion (Colossians 3:23-24).
However...not all will see the truth of Christ's teaching. They didn't see it when He walked on earth doing miracles before their very eyes...and many will not see even to this day. John says in v.37-40 "But though He had performed so many signs before them, yet they were not believing in Him. This was to fulfill the word of Isaiah the prophet which he spoke: “Lord, who has believed our report? And to whom has the arm of the Lord been revealed?" For this reason they could not believe, for Isaiah said again, He has blinded their eyes and He hardened their heart, so that they would not see with their eyes and perceive with their heart, and be converted and I heal them.”
This is why "the Son of Man" (Jesus Christ) "must be lifted up" (v.34); meaning death by crucifixion on a cross. Because death bring life...and His death brings life to ALL who look to Him for salvation. "While you have the Light, believe in the Light, so that you may become sons of Light (v.36)." Death truly is the way to life...life eternal!