In my experience, probably the most popular reason of a conflict is when someone (apparently unaware of what's important and what's not, or, worse, having his or her own opinion about the matter) tries to push their agenda, thus putting your interests in danger. That's what missionary's complex feels and looks like.
I don't know why (and probably the origin is not that important here, unless we regard a problem from neuropsychophysiological point of view) people (we) sometimes tend to feel their (our) job as a kind of a mission. It may be good when we want a good motivation. It is not that good when we need a team to function harmoniously. Meaning, if not altogether without a conflict, but at least without coming to a halt due to unresolvable contradictions between its members.