Denying Tenant's Request for a Roommate

[Albert Hoffman]

I have searched the forums and couldn't find this specific questions asked before . . .

A little background info:

I am living in one unit of a fourplex, renting another unit in the same building to a friend, and now he wants to move a mutual friend in as a roommate.  I'd like to avoid this situation, because I know it will lead to nothing but problems.

So, what justification do I have to deny a tenant's request for a roommate, other than "because I said so"?

Thanks in advance