I think it's much easier to impulse buy e-books on the Kindle than it is to buy a paperback. You get instant gratification -- you see a book you want, pay for it, and you get it without having to travel anywhere.

This is especially potent if you're already reading a series; just go buy the next book in the series. Sometimes I'll find out about a book and then just grab my Kindle to get the book before I forget about it.

Instant purchase, instant gratification!