At this time of the evening (Thursday night), the night before Jesus dies, his heart is troubled. He knows that he is going to leave his disciples, and he knows that they can't come with him. He wants to give them comfort to hold onto, even while he's not with them. This whole chapter is filled with that comfort. Jesus tells his disciples, that he is going to send a comforter- the Holy Spirit- to remind them of everything he has taught them- everything he is, and everything he has done.