If you have more than 1000 e-mails currently sitting in your inbox, then this posting is for you. Believe me, I've been there.
Here are some of the biggest reasons it happens:
1. I don't have time to read this now. I better save it to read later, just in case. (You probably won't. Ever.)
2. I may want to e-mail this person someday. I'll save his/her e-mail, so I can pull it up and just hit reply. (Why not just save their address in your address book now and call it a day. Kill the e-mail!)
3. I don't know why I'll ever want this e-mail again. But just in case, I'll hang onto it. (Kill it!)
4. These newsletters are interesting. I don't have time to read them. But, maybe I'll have time to catch up on the backlog and read them later. (When did you read that last gardening newsletter? If you can't remember, then it's time to hit "unsubscribe". If you think that you really will read them someday, move them to a folder).
5. I may need to document this conversation some day. CYA! (question: what are the chances? Solution: folder)