Joy is inextricably linked to our love for others. We cannot possibly experience joy when isolating ourselves from the rest of God's people. That's because we cannot experience God's love when isolated from the love of God's people, or when holding unloving attitudes towards others. Sounds harsh, but that's the basic truth as Jesus himself puts it. Hold a grudge, refuse to extend yourself, hide within a little shell around your life, and you will find joy a scarce commodity.
Love only can grow when it is shared.
Give love away and you will find it growing in your own heart. Perhaps first a bud, then a little seedling, but eventually a vigorously growing and healthy plant of joy. It doesn't mean that all will be roses. Just look at the setting in which Jesus speaks these words. They are part of a long speech he makes, which really begins in ch13 of John and ends in ch.18.