There are plenty of ways to rationalize it. Like: it's something you want to read NOW. It's on sale. Nice to have choices when deciding what to read next. Etc.
Then there are the sub rational reasons, which may be the real driver for most of our behavior.
Like: Hoarding instinct. It makes sense when you come across something to collect it as you don't know if it'll be there when you need it. From there, it is easy to lapse into just acquiring things.
Like: being over optimistic in estimating how much you can read, or for that matter, how long you'll be around to read it all. Maybe it's a hedge against mortality. (I always think of the scene in Moonstruck where the Olympia Dukakis character tells a womanizer something like "you know no matter how many girls you sleep with, you're still going to die, right?")
Like: we're socially conditioned to take what's given and then more. Up until age three or so, children will just stop eating when they're full. If you try to give them more food, they'll turn their face away or spit it out. When they get older they'll tend to eat what's put in front of them. A good way to trick yourself into eating less is to just use smaller serving dish.
Like: your brain digs it. We're always hearing how X is like heroin. Chocolate is like crack. Shopping is like crack. Which is ridiculous and of course not true, except in so far as chocolate and crack both light up some of the same reward pathways in your brain. Point and click shopping just makes it too easy to get that little dopamine hit. Having it pop right up into your reader? Potent. And if that isn't enough for you, throw in 3G so you can get your fix anywhere any time
