Why do Christians need to promote their religion to non-believers?

Door to door Jehovah's Witnesses / Mormons, people preaching in town centres, posters outside churches, alpha course. Why do Christians have to promote their beliefs to other people? What difference does it make to believers if other people do not believe the same as them? Do they not think they may offend atheists? I am interested to hear from people who actively promote their religion to non-believers and what drives them.