This is because either You or your computer, somehow, is Signing you Out of LR.
When you start LR it first opens a dialog where you must Sign In with an Adobe ID. That is an email address and password you used on the Adobe website to create an Account with Adobe.
Once you Sign In another dialog opens asking for the serial number of the product you are trying to register. Once you type in a Valid serial number and it is OKed with the Adobe validation servers you are good to go as long as you don't Sign Out of LR.
Now there has been some problems with Folder/File permissions on Mac computers where the registration doesn't get properly written on the hard drive and every time you start LR is ask all this same stuff over and over. There is an Adobe help page to try and fix this type of problems but I don't have a link to it. Google for it.
thanks just shoot me for responding quickly, but I seem to have the same problem as Mac users.
and I did the whole sign up thing before as you stated above.
I'm going to try to re install the program as I bought a cd.
If you ever had a subscription to the Photographer plan, or any subscription for Adobe software, you can't use that same Adobe ID for LR 6. That Adobe ID is, Would be, Is Always, tied to a subscription plan even if it become inactive, IE you have canceled it or stop paying. It must be an Adobe ID that was never associated with any subscription plan.