When properly looked at, natural leadership occurs when a dominant person acts in his/her own self-interest and, in the process, enables followers to benefit from his/her self-serving behaviour. In ancestral times, as a silly and simple but relevant example, perhaps it was a man who knew where the watering hole was, and wanted to get there anyway. Followers who were less competent in finding it could help him carry his goods while he led them there. This natural process selects for competent, capable leaders who could articulate to followers how, by being subservient and giving up some autonomy and self-interest, they could gain something from his capabilities. Leadership and followership evolved because it was an efficient process that eventually led to a net advantage for whole groups over the course of social history, both human and animal.
Servant leadership is the idea that leaders should want first and foremost to serve followers and are held accountable by the people the leader governs, in pure altruistic fashion. Like many other modern good-to-haves, servant leadership is unlikely to have emerged simply from rational thought alone - its formulation was forged from a drastic need to deal with the fact that the influential ability of leaders meant that leaders could also get away with many dastardly things. The evolution of leadership wouldn't be complete without a counter from followers, who adapted by evolving emotions that made them upset, angry, cheated and disgusted when leaders become corrupt. Servant leadership is a socially constructed dynamic that actively suppresses the tendency of leaders to put their own interests ahead of their followers' interests.
But just as much as we have those emotions acting as alarms to warn us against being too unfairly taken advantage of, people far and wide still see confidence, dominance, assertiveness and aggression as virtues in leaders. When we can potentially see these traits as attractive in a leader, we allow him or her to get away with doing whatever it takes to "get shit done." A tasteful mix of these characteristics in a person makes him or her charismatic, and we often forgive charismatic people for taking a little more than their fair share of the pie.
Because at heart, the propeling force of leadership is self-interest. Lots of it. A person wouldn't last as a leader without it, and simply bestowing a fancy title on anybody doesn't make the person a sustainable leader. This is the reason why a student studying politics doesn't necessarily become a politician after that; governors, CEOs, directors, pastors and generals are a self-selecting lot and, for better or worse, their immense, self-aggrandising belief in the value of their own ideas, goals and needs over others' persistently drives their rise to the top.