When I log into Combine I check the "Stay Signed In" Button before I log in, but later when I turn off the web browser and later when I turn it back on I have to write my information(Handle and Password).
"Stay Signed In" puts a cookie on your computer that holds your username and a special key to authenticate yourself. It should keep you logged in for a few months.
Things like antivirus software or privacy extensions might delete the cookies. If they are deleted, obviously you have to sign in again. Check to see if this occurs when you have turned anything suspect off, and then slowly turn things on to see what causes it.