When the disciples found out that Jesus was going to be betrayed, and then killed, and then rise again after three days, they were extremely sad. Why?
I think the first major part of this verse is the fact that the disciples believed Him, at least to a degree. If all of the miracles until then were just exagerrations (Jesus fed 5 instead of 5000, for example) I don't think the disciples would have believed Him now.
I said that they believed Him to a degree. They were really sad that Jesus was going to be betrayed/killed. So why were they still sad, even after Jesus said He would rise again?
Maybe it was because they were sad He was going to be betrayed and killed in the first place, because maybe they realized they couldn't help Him.
Or maybe it was because they only had enough faith to believe what humans think is 'possible'. They would believe that He would be betrayed and killed, but rising again was a little out of their belief.
(Response Question: Why do you think they were sad, even knowing that He would rise again?)
One thing is for sure, though. The disciples obviously loved Him and trusted Him, enough to be sad when He said He was going to die. Therefore, Jesus must have done something to earn that love and trust.
In His Service,