On the Mount of Transfiguration Moses and Elijah appeared and began to discus Jesus' departure. Why Moses and Elijah? These men have two things in common: one is that neither one of them completed the mission God gave them.
Moses was prevented from taking the Israelites into the Promised Land due to his display of anger.
Elijah had to be replaced by Elisha due to his belief he was the only person living for God. He had given up on Israel.
Could it be that these men were uniquely able to encourage Jesus to complete his mission because they knew what it was like to fail God?