For security reasons, the system automatically logs you out after a few minutes of inactivity. You might be busy typing a long message, but since there was no interaction with the system, it logs you out.
To prevent this from happening please check the "Remember Me" box below the username and password box whenever you login. Also, don't log out when you are done browsing. That way, you don't have to login the next time you visit the site. The system will remember you for a 14-day period.