John Piper, in his book, Let the Nation be Glad!: The Supremacy of God in Missions, clues us in on the BIG reason for missions.
"The goal of missions is the gladness of the peoples in the greatness of God."
(Let the Nations be Glad:The Supremacy of God in Missions, John Piper, Baker Academic, Grand Rapids MI 49516-6287, 2003)
Now that is a big reason! Let's reflect on this goal of missions.
1) To accomplish this goal the missionary must first see the greatness of God for his(her) self.
A missionary who has solely a desire for the salvation of souls will surely become depressed and discouraged when he sees that not many come to faith in Christ. But the missionary who desires new converts in new peoples to experience the joy and greatness of God will not be disappointed. The salvation of souls is not bad, but it is not the primary reason for missions. The primary reason for missions is the gladness in the greatness of God. This requires the salvation of souls to be sure.
The missionary must be enthralled with the greatness of God himself, in order for him to point people to the greatness of God. A man cannot give directions to a place he has never been. It is equally true that a missionary can never point people to gladness in the greatness of God if he has never been there himself.
2) God desires joyful worship from all peoples. (Read Psalms 95-100) Sing to the Lord, all the earth! Declare His glory among the nations! The Lord reigns; let the earth rejoice; let the multitude of isles be glad! The Lord reigns; let the peoples tremble! Make a joyful shout to the Lord all ye lands! The plan of God is to have people from every nation, tribe, langauge, people, and family find their gladness and joy in Him.
3) The goal will be realized. In the book of Revelation we get a glimpse of Eternity. All nations, tribes, peoples, and tongues are standing before God worshipping joyfully the God of their salvation. This should give us boldness and joy in our missionary endeavours. To know that we can bring joy to people, that we can give the Gospel to them, and to know that we will be successful, because of God's written Word, is a pretty good incentive, don't you think?!