Partizion uses browser cookies to store a secure token that allows you to stay logged into Partizion. When this token expires (Or doesn't exist) you are logged out. Sometimes your browser settings clears all cookies, including the Partizion cookie, when you close it. This means that there is no way to remember your user credentials and you will have to log in again.
To fix this, simply check that your browser settings are not clearing all cookies whenever you close your browser, which is probably quite often.
In Chrome, go to the Settings Page. From there, scroll down to the heading: Privacy and Security and select "Cookies and other site data". Make sure that the setting, "Clear cookies and site data when you quit Chrome" is disabled.
Now you should be able to stay logged in to Partizion for up to two weeks. Happy Tabbing!
You can always manually logout of Partizion, or even clear your browser cookies whenever you like.