Why do you always use my email address on all my domain registrations instead of my customer's email address? They want their email address listed as admin contact.
This is up to you - you can make changes to your domain registrations here:
However, keep in mind that there are renewal notices sent out automatically to the email address listed for the admin contact for the domain - you probably want to keep this as your email address so you can get these and other notices instead of your customer. Often they will just delete these, figuring "they handle this for me" then when the domain is disabled for non renewal, they are furious!
It's also good to keep the email address as yours in case the password is changed and then lost - often the customer changes their email address frequently, and the password retrieval system can only send to the admin contact. If the admin contact email address is invalid, there is a long complicated fax authentication process to go through.