Firstly, have you clicked the 'remember me' button?
Unless you're using a public computer (ie in an internet café), then once you've logged in, you should never need to log out again!
If you have clicked the 'remember me' button, then it's probably your computer that's at fault, in that it's got a problem with 'cookies'.
I don't know what computer, operating system or browser you run, but if you do a search on the internet for your particular set up and 'clearing cookies' this should solve the problem.
This isn't the first time people have had this problem, but in every case so far, it's been down to an individual user's browser - not the forum software. Don't forget if it was a problem with the forum software, then everyone else would be suffering the same problem!