Well it all starts out with having the Gospel in our lives. When we know how this Gospel has blessed our lives we voluntarily go on a mission paying all our own expenses to share it with others. We sacrifice two years to be able to bring other families together for eternities. We have left our families for 18 months(sisters) or 2 years(elders) to go and share this Gospel which has blessed our lives. This is the motivation behind most LDS Missionaries.