How long has it been since you really thought about all the things you subscribe to in email? The newsletters, the notifications, the email lists. It's so easy to delete the old stuff when it comes in that you might be used to doing just that - deleting instead of unsubscribing.
I've spent the last couple of weeks unsubscribing from everything and I was amazed at the sheer number of things that came in, but always went unread. My guess is there were 25-30 different newsletters and updates. I had no idea.
My advice is to be aware and remove the things you're not reading if you have a choice. It's quite liberating. Now, if I can just figure out how to unsubscribe from spam, I'll be all set.