Because this commandment can be fulfilled in only one way – by abiding in the vine that is Jesus. It is Jesus who chooses his disciples, so there is no room for personal pride. And it is Jesus who shows love to the other person through his disciples, so there is no room for self-identity. Jesus directs his disciples, and gives them the impulses to reach out to one another, and it is his love abiding within that is willing to die for them. Jesus bears the burden of this commandment, and he makes the sacrificial love possible for his disciples to show, which is the most effective form of testifying.
And that's where the joy comes from! We FEEL Jesus' presence within us, when he loves others through us, especially when we don't really like them. We KNOW Jesus is abiding within us, when we are guided to perform acts of self-giving love. The joy we receive is not a joy of accomplishment, because it's not our accomplishment. It's not some kind of twisted joy-in-suffering, because OUR suffering is not the suffering that's important. We receive Jesus' joy, the joy of Jesus-in-us, and this joy is for us...complete. This joy does not change with changing circumstances. This joy does not depend on our abilities. This joy cannot be stolen by anyone or anything. And the joy of Jesus' presence will be ours forever.
Gracious God, we ask you to give us everything we need to bear fruit that will last, loving one another as Jesus loved us. Help us to always abide in Jesus, as he abides in us, and enable us to recognize the joy we have in the presence of Jesus Christ, our Saviour and Lord. Amen.