I had this conversation with a fellow missionary and we were sharing about how we’ve survived this far. (Actually in reality we are not just surviving, we are in fact thriving in the joy of the Lord.) That I can say for sure! Why? How can we be happy in the midst difficulties? Difficulties are everywhere; the important thing is we find what kind of life we want to live.
I think missionaries are happy because they found what they wanted. I think, other people who are also working in other ways can be happy too if they find there work as the path God wants them to be.
Another thing, what I found in mission is that we can be in desolation ourselves but can still be joyful because we find our consolation in the joy of others. We rejoice of others joy and that is enough consolation for us.
Lastly, our joys are grounded in our faith in Christ, so things can be fleeting but we can be joyful because His is eternal.