That's a pretty big question so I can only give a pretty general answer. Humans of course are social animals so naturally most of us live in groups. Even fir those that try to avoid this the infrastructures we have become reliant on and growing populations force the majority of humanity to play a wider part in society. So by and large we must function as part of increasingly large groups. If we are to function as a group, particularly in regard to others but also internally, we need to be able to make decisions as a group. In such large numbers a pure democracy just doesn't work, even if it was practised and views were balloted in as opposed to votes those interpretating the views would have the power. Leadership also gives advantages, for example being better able to present a united front against invaders, internal security, better organisation of resources. All of these things benefit from the larger view given by a more centralised leadership.