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.
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, which is not uncommon if you have a business, what you can do is set up a special alternate email address somewhere else, for example at a reasonably reliable provider of free email accounts, and easily 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 to get in touch with us, 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. Make sure that whatever setup of forwarding and responsibility you set up with multiple email addresses can be represented with a directed acyclic graph.