The 12 disciples had been hanging out with Jesus for the past three years. They witnessed His teaching, peace, and miracles. Maybe, in the beginning, they were a little unsure of who He was, but they should have been convinced that He was the Messiah by the end.
The disciples must have felt pretty special that Jesus would ask them to follow Him so closely. But, just a while before this part of Mark, they learned that He would die soon and that one of them would be the betrayer. At this stage, Jesus started to show worry for the first time.
Jesus' suffering started before the events leading to His crucifixion. It started right here, in the garden of Gethsemane. Jesus knew what would be happening to Him. He also knew what would be happening on a deeper Spiritual level. Jesus, who had never sinned, became the embodiment of sin. "He bore our sins in His body (1 Peter 2:24)" - Jesus was bearing the sins of the world.
This is why He was "very sorrowful - even to the point of death." This is why He asked God to "remove this cup" from Him.
Hebrews 12:1-2 says that there was "joy set before Him" that helped Him "endure the cross."