Why does sugar hurt a cavity so much?!

Probably because of it's intensity, and concentration. It would probably hurt the same if you ate a piece of rock salt and crunched it around that same tooth. Eat a candy bar, and all that sugar in the chocolate is concentrated, it gets up in the tooth, coats it, gets inside the cavity and coats the nerve ending, and of course that's going to cause pain. Eventually it will get worse.....soon it will start to hurt with temp variances, then eventually after that, it's just going to hurt all the time because the nerve is further exposed. Of course that usually happens at the worst time, like on holiday when all the dentist offices are closed. LOL