Simply because I'm sitting in Philosophy and I'm bored.
Can you imagine what it would be like to meet your idol, someone you put up on a pedestal out of pure admiration, only to find out that they were a fraud?
That they'd completely let you down.
For James Moriarty, that's what it was like meeting Sherlock Holmes. For years, James had studied Mister Holmes. He had read every last article Sherlock had written—right down to the boring ones.
He dreamt about meeting Sherlock; what he would say, jokes he could tell, even deductions he'd make himself, all in an effort to impress this one man.
So one can only imagine his disappointment when Sherlock didn't live up to his expectations.
Sherlock was ordinary.
Average.
Easily swayed out of love.
And yet, there was something about him. Something about his humanity that made him attractive, though—in Jim's eyes—made everyone else weak.
Jim promised Sherlock that he would "burn the heart out of him". And he did. He kept his promise.
But he did it purely because he refused to come in second place to a simpleton Doctor, or a Detective Inspector or a landlady. Because, even if he'd let him down, even if Sherlock had been ordinary, Jim was willing to compromise.
He could be brilliant enough for the both of them.