Immediately before these verses, Peter confessed faith in Jesus as the Promised Savior (Messiah or Christ). People had a lot of false hopes and expectations for the Messiah. So Jesus told the Disciples exactly what He had to do. Other Gospels report that Peter objected. Luke did not record that. Luke recorded Jesus first prediction of His suffering, death and resurrection to forgive and restore us.
People still have misplaced expectations for Jesus, the Christian faith and life. They expect to be happy and prosperous. Jesus has a different idea that we will look at in tomorrow's reading from Luke 9.