Why does Holmes find the crime in The Hound of the Baskervilles interesting?

I presume that the crime in The Hound of the Baskervilles to which your question refers is the mysterious death of Sir Charles Baskerville, recounted by his friend and neighbor Dr. James Mortimer in chapters 2 and 3 of the work.
It is entirely likely that Sherlock Holmes is excited (though perhaps excitable would be a more accurate word for his reaction) about the case for two reasons. In the first place, he is well-known for being a man of science whose "methods" consist of making inferences based on empirical evidence; indeed, he is a self-described "specialist in crime" (chapter 1), and the case seems somehow to involve the supernatural via the legend of a giant, ghostly, malicious hound. It is a question that would naturally fascinate as rational a thinker as is Holmes, at least until he could prove a thoroughly rational solution.
In the second place, Holmes's fascination with the case is doubly underscored when Dr. Mortimer, whom Holmes calls "the man of science" and "a man of precise mind" (chapter 1) upon meeting him, expresses first his admiration for Holmes's being "the second highest expert in Europe" and "a practical man of affairs," appeals to Holmes for help, and intimates that his own doubts about the legend having come true cause him to "not know what to believe" (chapter 3). For a "trained man of science" (chapter 3), as Holmes calls Dr. Mortimer, to doubt his own education and rationality in the face of a frightening legend intrigues Holmes, who seems never to have been tempted to abandon his practical scientific methods and never to have met a case that could not be explained rationally. But if the doctor can be swayed, Holmes wonders, then what kind of case is it, really?
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