What Makes The Greats Great?

There are a lot of things we consider great. The works of Da Vinci, Picasso, Monet, Renoir and so many other painters. The writings of Tolstoy, Steinbeck, Sartre, Shakespeare etc. The music of Beethoven, Mozart, Chopin. The voices of Billy Joel, Lata Mangeshkar,the Beatles. The speeches of Martin Luther King, Abraham Lincoln, Barack Obama and now, Narendra Modi.
So what is the criteria one must fulfil to be considered great? What made Mahatma Gandhi a great man? He did what no one else could. He gave his country freedom - without raising a single weapon other than his voice and his principles. What is it about O'Henry's short stories that they're read and thoroughly enjoyed even today? What is it about Pablo Picasso's paintings that compels you to stare at them trying to comprehend the thoughts behind it? How about the Mona Lisa? Why is that great? It's a tiny painting, of a not so beautiful woman, in a museum that's filled with several other infinitely more beautiful paintings. But tourists visit the Louvre solely to see the Mona Lisa. To figure out the mystery behind it. To know why Leonardo da Vinci said it was his masterpiece.
What is it that makes one great? Is it a lot of people saying its great? I think the X-factor that gives it the oomph quality is something that is different for everything and everybody. Gandhi was great because he had the courage to fight a war the way he wanted. Shakespeare's plays never fail to make one laugh, cry, make the hair at the back of one's neck stand up. To Kill a Mockingbird is a timeless classic because it's written in the simplest language possible and yet talks of something so deep and profound.
Being great is having the courage to be different. Even if you are risking ridicule from society. Most great men and women were not recognised for their wisdom and talent during their lifetime. Its one of the tragedies of being talented and intelligent- your audience consists of people who may never understand you.
So, I may not understand why something is so great. But I can't judge it unless I've seen it for myself. There's a reason why its great. Maybe we get it, maybe we don't. One of life's great mysteries - only for us to solve.