That's why they were sorrowful. His going away made them sorrowful. It was His Father's will. The Father is greater than Jesus' human nature. Furthermore, their lack of faith in Jesus made them unable to keep His Word.
Faith is fruitful.
Unbelief is fruitless. Jesus stresses the need for faith in Him and His Word. So were the disciples. But Jesus did not cast them out. This results in God living in the believer.