Peter loved to be a disciple of Jesus but when he heard Jesus talked about what was required of a true discipleship Peter stepped back. Jesus unfolds the cost of the discipleship and Peter couldn't comprehend. Jesus talked about His own cross and how those who follow Him must do the same and take up their cross and follow Him. Like many of us, Peter wanted to follow but denied the cross but that is not the way of Christ.
Without the cross and the resurrection the way of Christ and the ultimate loving sacrifice is just an empty promise. Dying daily is a part of the living and all other earthly living things must die one day. Dying means to give life for others. We die daily, bit by bit. Daily sacrifice means many things. It could involve the whole person or a part of the person. It means surrender our selfishness or desires for the benefits of others. It could be to give up of our will and voice for unity and peace. It could be strict control of bad habits and vices for personal growth. It could be to conquer pride and arrogant for the spiritual nourishment. It is up to us to decide what way we want to pursue: the way of Christ or the way of the world. If we choose the way of the world it means our little daily deaths would die forever but if we follow the way of Christ, our little daily deaths will live forever because the resurrection of Christ will incorporate our daily deaths to His and rise with Him to new life forever.
Christian believes the eternal life matters more than temporary earthly lives. Denying the existence of the eternal life means a person would care more about the present life. Their absolute view about eternal has its foundation on human knowledge. We make our claim not on human knowledge but on the foundation of the teaching of Jesus who made claim that
I am the way, the truth and life and those who follow Him will have eternal life. Jn 14,7
Confessing Christ is the Messiah was the initial step of the commitment to follow. Following Jesus requires taking the same road and obeying His teachings. There is no clear road map mark to follow. We grope in the dark to move forward and backwards as Peter did. Mistakes are made because we value things based on human knowledge and some of the human standards are the stumbling blocks for us to follow the way of the Christ.
Just a few minutes before Jesus praised Peter for his wise answer and gave him the key to the kingdom then a few minutes later Jesus rebuked Peter for stopping Him to carry the cross. Peter who allowed no harm to come to Jesus was an act of love and of good will but Jesus told Peter that his view although it appeared to be good had came not from God but purely human and it was going to interfere with God's plan of salvation. Peter slips back to the human standards that involved two mistakes. Firstly he fails to understand the mission of Jesus that entails the act of laying down His life for His flock. Secondly, Peter fails to know that all future shepherds will be doing the same.
Laying down one's life for the flock has nothing to do with religious extremism but it is simply an act of showing what the true and unselfish love must involve.