In Alaska you can find the world's biggest mountain, if you measure from base to peak. The Mountain is known as Mt. McKinley or Denali, depending on your cultural loyalties. The Mountain is so big that from hundreds of miles away, it looks like the only mountain in the world. That is, if it will let you see it. Locally, in Utah, south of Salt Lake City, there is a mountain known as Timpanogos. It stands out as being one of the bigger mountains along a chain of mountains. It also sports it's own glacier, and has snow on it more often than it's neighboring mountains do.
What both of these mountains have in common is that each one creates it's own weather. In both cases, you may not have a cloud in the sky otherwise, but a small cluster of clouds near the mountain. The exact weather of these mountains is unpredictable, the mountain will do as it chooses.
The same thing is true of people. Human behavior is somewhat predictable, as is the weather. Every now and then, a great person steps up and changes the weather in unpredictable ways. By choosing to be a great person you will have the tendency to change the weather around you, to create weather even.
What is greatness? I'm still figuring that one out. What I know so far is that the great don't generally worry about the goal of being great. They don't try to be great. Instead, they go out and do great things.