There are two possibilities here... 

The site will log you out if you don't change pages or click on refresh once every 15 minutes.  This is a security feature that keeps members from accidently staying logged in while they go to bed for the night, go to dinner... etc. 

You will know this automatic logout is coming because a warning window pops up telling you to please change pages or click on "refresh" soon.

The less obvious reason you would get repeated login requests is related to Norton Internet Security software.  To put it mildly, there's a problem with Norton's software.  If you type "Norton Problem" into the Google search engine, you will find nearly 4-million referneces to this well-documented bug.

For info on what to do, here's a link to the Norton site help area...

http://service1.symantec.com/SUPPORT/nip.nsf/pfdocs/2000031311301136