From http://www.businessinsider.com/munger-explains-buffetts-success-2014-9 :

So what exactly did Munger identify as some of the unique keys to Buffett being "so unusually successful"? He began by noting:

The first factor is the mental aptitude. Warren is seriously smart.

Munger continued by noting Buffett "out-achieved his mental aptitude." Then there's the good effect caused by his doing this since he was 10 years old. It's very hard to succeed until you take the first step in what you're strongly interested in. There's no substitute for strong interest and he got a very early start.

Warren is one of the best learning machines on this earth. The turtles who outrun the hares are learning machines. If you stop learning in this world, the world rushes right by you. Warren was lucky that he could still learn effectively and build his skills, even after he reached retirement age. Warren's investing skills have markedly increased since he turned 65.

Munger notes this reality is "really crucial," because he suggests, "having watched the whole process with Warren, I can report that if he had stopped with what he knew at earlier points, the record would be a pale shadow of what it is."

And Buffett hasn't just continued his learning, but he's also seen all of the steps laid out on the path set before him as a chance to gain better understanding. Munger compares Buffett to the Roman emperor Marcus Aurelius who "had the notion that every tough stretch was an opportunity — to learn, to display manhood, you name it. To him, it was as natural as breathing to have tough stretches."

As a result, Munger suggested, "Warren doesn't spend any time on self-pity, envy, etc."

Put simply, Buffett has continually dedicated himself to refining and expanding his understanding of investing.

The key to remember

Warren Buffett once said:

I insist on a lot of time being spent, almost every day, to just sit and think. That is very uncommon in American business. I read and think. So I do more reading and thinking, and make less impulse decisions than most people in business. I do it because I like this kind of life.