Missionary zeal does not grow out of intellectual beliefs, nor out of theological arguments, but out of love. If I do not love a person I am not moved to help him by proofs that he is in need; if I do love him, I wait for no proof of a special need to urge me to help him.
Once you discover that you can, then you must. And it's not easy. You have to take direct steps. You really have to count your blessings and you have to make a decided effort to not get seduced by the blues.