Not that I receive an avalanche of fan mail, but I hate to send back a reply that sounds too mechanical -- and there are only so many ways of saying that you're glad that someone likes your books so much. That doesn't make the mail any less welcome, but it can slow down the response time, as I try to think of something to say that doesn't come across as too much of a form reply. (Then I feel guilty because so much time has elapsed, and I end up just saying the same sort of thing I usually say anyway.)
If someone asks an interesting and original question about something I've written, they're more likely to get a longer and (I hope) more interesting response.