The biblical motive for mission is to be joy, gratitude and excitement. Christ is risen. The first missionary movement was born out of persecution (Acts 8:1), but the disciples involved in it scattered in all directions, telling people the good news about Jesus (Acts 8:4).
Declaring the good news about Jesus is the only point of missions, and consequently other motives are not just inadequate for the task, but are rather an utter contradiction to it. The gospel as it is in Jesus is a gospel of sheer grace, and to be guilted out to the mission field is to deny the efficacy of that grace. And if the missionary doesn’t believe the message, then why should anybody else believe it? And to go to the mission field in an attempt to get one’s own spiritual life in order is likewise a denial of how God works with men.
Joy is the only motive consistent with the message itself.