individual greatness

what makes an individual 'great'? i mean, what is it that drives people to reach a point where they realize their potential? there are so many human beings that walk on this earth who will leave a worldwide legacy and then others who seem content to just exist and die. i guess 'greatness' is very subjective, but we cannot deny that the mandela's, mother teresa's, gandhi's, & mlk's are exceptions to the norm.  regardless of their backgrounds, they were able to rise above & out of the typical patterns of thought in their 'tribes'. are some people just born this way? with a sun so bright inside that they must light a path? or can this force be acquired through life experience? i am not so disillusioned to think there is a clear answer.  as with most questions in life, there are probably a myriad of factors that play into the answer.  any thoughts?