John wrote: We are writing these things so that our joy may be complete (1 John 1.4).
It seems to me that the missionary spirit is one which finds its joy only in the salvation of others. It is not happy to see that others are not enjoying, same as oneself, the benefits and pure bliss which a life in fellowship with God in Jesus Christ offers to all. John wrote because the joy of the apostles depended upon the fellowship of his audience with the Holy Trinity; only this would complete their joy, only this would satisfy them, only this would give them perfect happiness.
When you have a good thing, you want to share it with others. This is how I am: when I discover a beautiful piece of music, or an impressive band, or watch a great move that really profoundly impacts me, or read something wonderful, I want to write about it and tell others so that they can enjoy it, too. And this is why I went into seminary in the first place. I began to know Jesus, and to enjoy fellowship with him. I was learning wonderful things from the Bible, things which deeply and profoundly impacted and helped me in my own life, and I wanted others to taste and see for themselves.
For the same reason, it makes me sad when I interact with friends and acquaintances and strangers who don't have the same joy that I have, who don't love Jesus Christ and who seem not to care about the great sacrifice that he made on our behalf, who have no sense of awe and gratitude and joy at the salvation of God provided for us in his Son. What I have, I want them to enjoy as well, but they don't. I love Jesus Christ very deeply for what he did for me, and I want others to love him, too, and to reciprocate the phenomenal and incredible love which he has for us. But still some don't do this. I pray for them, I hope for them, I petition God for them.