Why does Jesus call Peter Satan?

Satan is actually the Ego of the individual. The Ego is all the defects one has such as, Gluttony, Anger, Lust, etc. Furthermore, the Essence one has is perfect Matthew 5:48 "Therefore you shall be perfect, just as your Father in heaven is perfect." This is what Jesus meant when he said Mark 8:34 "Whoever desires to come after Me, let him deny himself, and take up his cross, and follow Me."

Deny himself - Eliminate the Ego

Take up his cross - Creation of Soul

Follow Me - Sacrifice for others

Jesus realized that the devil began to direct peter's Words and have no regard for what Gods plans were that's why in verse 23 he goes on to say "You Are not mindful of the things of God, but the things of men."