30 And now, behold, my joy is great, even unto fulness, because of you, and also this generation; yea, and even the Father rejoiceth, and also all the holy angels, because of you and this generation; for none of them are lost.
31 Behold, I would that ye should understand; for I mean them who are now alive of this generation; and none of them are lost; and in them I have fulness of joy.
In asking the question to my wonderful wife, she offered several thing in relation to our obedience to Christ's commandments and remembering him. When I said that Jesus himself encapsulated all that she had said into a simple statement, she of course wanted to know this statement.
This is what he said, not just once but twice in two verses: "...none of them are lost...".
In a world crammed full of distractions, numberless causes and concerns on which to exercise one's energy and virtue, the Messiah simply wants each of us not to get lost. In my mind, the statement brings up the concept of Christ as the Shepard and all of us as sheep - we need to stay with the Shepard always, so that we don't wander off and become "lost" to him.
Jesus is a constant - he is always there. If someone gets "lost", it is our leaving Christ and the principles that he taught and the ordinances and behavior that he commanded of us. When we choose to follow the Savior, it gives him joy, as he brings us closer and closer to salvation and our Heavenly Father.