If you're receiving a login prompt each time you open Outlook, you probably have the option to always prompt for login credentials enabled.
Below are the steps to change those settings.
Click the File tap to the left of the Home tab in Outlook.
Click on the Account Settings button.
Then click on the Account Settings... link from the drop down.
Double click on your account.
Click on More Settings on the next screen.
Click on the “Security” Tab and uncheck the box that says “Always prompt for login credentials”.
Then click OK, then Next, then Finish.
This should correct the issue with you having to log into Outlook every time you start it up.