Do you use multiple profiles in Outlook? If you go into Control Panel, Mail and select your most used profile in the Always use this profile dropdown, then change it to Always ask... the profile listed when you first start Outlook will be the one you selected and you'll have the option of choosing a different profile.

If you prefer to use shortcuts to open Outlook, you can configure it to open Outlook using a specific profile. Use the command line switch /profile profile_name and replace profile_name with your profile's name. This also works with the Start menu, Run command. Type outlook.exe /profile profile_name in the run box to open Outlook using a specific profile.