We need you to have a current, working email address at all times in case we need to contact you about problems. If anything important to your contact email address is hosted here (DNS, domain registration, or email forwarding) and there's a problem with your service, we might not be able to contact you. That's bad.
For example, suppose your domain is registered here and it expires. And suppose that you forgot your password and need a new one mailed to you to fix things. But you can't get that message because your email doesn't work when your domain is expired. In cases like that, our Privacy Policy turns from a critically-important protection into a colossal roadblock; you'll wind up giving us a DNA sample and fingerprints to get your access back. (Well, not really, but it'll feel like that, especially if you're in a hurry.)
These types of circular dependencies create a lot of headaches, so we simply don't allow them.
If you don't want to have multiple email boxes to keep track of, there is a workaround. If your primary email address is in the domain that you want to host here, you can create a special alternate email address somewhere else, for example at a reasonably reliable provider of free email accounts, and set it up to forward copies of all the messages it receives to your normal mailbox. That way, you'll get all the messages, but if there's ever a problem, you can fall back on the alternate mailbox, secure in the knowledge that it's got copies of any recent messages we've sent you to work from.
Just be sure that you don't need your primary email address to reset your alternate one, in case you forget both. A bigger circular dependency is still a circular dependency. Whatever setup of forwarding and responsibility you set up should be able to be represented with a directed acyclic dependency graph.