Ye have heard how I said unto you, I go away, and come again unto you. If ye loved me, ye would rejoice, because I said, I go unto the Father: for my Father is greater than I.
Jesus had already told His disciples that He was going to leave them, and then later He would return to take them to heaven with Him. If they loved Him, this would have caused them to rejoice. The disciples could only love Christ as much as they understood who He was, thus their love was not as great as it should have been.