You can also Digitally sign your file, and if the user sets up their computer to accept your signature, Macros will be enabled automatically for the user on that computer every time they open something signed by you.
You can also have the user lower their Macro Security level, though that is not recommended, as it could allow viruses in.
Basically, there is no way to do it automatically, which is actually a good thing. Otherwise, people could force viruses on your computer pretty easily!