Interestingly, we find that Jesus was troubled, himself, just a short while before as he spoke of Judas, who would betray him. A short time later, as he was still addressing his disciples, Jesus tells them not to be troubled!
The difference is that Jesus knew what was coming for him! The disciples had no clue. Jesus knew that he would handed over to the Roman authorities, tried, mocked, spit on, beaten, scourged, made a public spectacle, nailed to a cross and die there – in just a matter of hours.
The disciples still didn’t understand what Jesus was getting at – what he had been getting at for a long time. Jesus had said similar things many times before. He not only predicted his death, but his resurrection as well. But, his disciples never did realize what he was talking about (until after the fact).
Think about that: the disciples, who lived with Jesus for three years and spoke intimately with him often, didn’t get what he had been talking about throughout that whole time they were with him. They didn’t understand. They needed something more than what Jesus, who was God in the flesh, could give them!