Why did Jesus tell the rich young ruler about eternal rewards when he asked how to obtain eternal life? He couldn't be given an answer that he could assimilate unless the issue of goodness was settled first.
Addressing Jesus as "Good Teacher," the man asked, "What shall I do to inherit eternal life? And Jesus said to him, 'Why do you call me good? No one is good except God alone." (Mark 10:17-18)
Then Jesus probed him on the question of his own goodness. "Do you realize that no one is good but God?" (vv.19-20) This man thought eternal life was something that could be obtained by doing good. (Mark 10:17) He thought he was good as revealed by his self-righteous boast regarding the commandments Jesus quoted. But the young ruler asked Jesus, "What am I still lacking?" (Matthew 19:20)
Jesus said he did lack something. But, in an effort to penetrate his self-righteous facade, Jesus told him to sell his possessions, give to the poor and follow Him and he would have treasure in heaven.
That isn't the same as believing in Jesus. That isn't the message of John 3:16. And the consequence of doing that was not eternal life; it was treasure in heaven. His response showed that he was selfish, materialistic and earthly minded. He needed to trust in Jesus Christ for eternal life.