Mormons go on mission ostensibly for the same reason that Catholics go on mission, albeit for a very different version of the Gospel. In their Book of Doctrine and Covenants they claim that Jesus said, “proclaim my gospel from land to land, and from city to city… bear testimony in every place, unto every people” (Doctrine and Covenants 66:5,7). They will also refer to the biblical mandate of Mark 16:15-16 “Go into the whole world and proclaim the gospel to every creature. Whoever believes and is baptized will be saved; whoever does not believe will be condemned.” According to their own website, there are currently 52,000 Mormon missionaries serving in 350 missions around the world.
While it is not officially mandatory, it is an unspoken expectation that every young person aged 18-22 who is able will go on at least an 18 month mission. Quite a bit of pressure can be applied to do so. Depending on the area, if a young person refuses to go on a mission the social and ecclesial consequences can be pretty severe for him/her as well as for the rest of the family.