It's easier to leave than be left behind. Sherlock Holmes had always doubted the veracity of that statement.
He knew firsthand how hard it was to leave... to ignore Watson's cries at the edge of the Falls. As he hid in the Continent under all aliases, the detective thought having left was the most difficult thing he- or anyone- could endure. However, when he came back to London, to Watson, a seed of doubt planted itself in his brilliant mind. He saw how harshly the three years had taken their toll on his friend. He knew then that being left behind surely wasn't easy.
Twenty years later, he finally understood. As soon as Watson's train left the platform, he realized it was far, far easier to leave than be left behind. And that it was almost more than he could bear. The only thought that kept him going was the ever-so-slight chance that Watson's hiatus would only be as temporary as his had been... being left behind was a veritable hell.