"From that time on, Jesus began to show his disciples that he must go to Jerusalem and undergo great suffering at the hands of the elders and chief priests and scribes, and be killed, an on the third day be raised."
It's human nature. Animal instinct. "Protect thyself." Look out for #1. Or, in the immortal words of the BeeGees, "Ah, ah, ah, ah... stayin' alive!" We seem to be wired towards self-preservation. That's probably why Peter had such a hard time with what Jesus had said to them. "God forbid it, Lord!" he cried out. "This must never happen to you!"
The fact is, they'd just gotten going. Three years seemed like such a short time to be in ministry together. Jesus' momentum, following, and fame had really taken off lately! And now, Jesus starts talking some nonsense about suffering & death?!?! Doesn't he get it? That can't happen to The Chosen One!
But Jesus knew differently. As humans, we may be prone to self-preservation... but we were meant to live a life of SELF-SACRIFICE! Yes, it was counter-intuitive and counter-cultural (it still is!)... but the Kingdom of God is based on putting others first... and responding in sacrificial love! Jesus was trying to help them learn the powerful lesson that this kind of lifestyle actually leads to a more abundant & meaningful life! The disciples had a hard time learning that. So do we.