Hello, thank you for reaching out to Microsoft Q&A.
May I confirm whether you are accessing your email using the Outlook desktop application or Outlook.com in a web browser?
When you sign in to the Outlook desktop application with your Microsoft account, the app securely stores a sign-in token after you authenticate. As long as this token remains valid, Outlook will not prompt you to enter your password again. This behavior is by design and cannot be changed in the desktop application.
(The sign in token only becomes invalid if you make any security changes such as password reset, signing out from your account, clearing Windows credentials, etc...) Since these changes happen automatically in the background, it is not something you or any user can realistically keep track of each time. Also, this behavior cannot be changed using the built-in settings of the Outlook desktop app.
If you access your email through Outlook.com in a web browser, the browser may prompt you to sign in each time if it is configured to clear cache and cookies when it is closed or restarted. Once this data is cleared, you will be required to enter your password again the next time you sign in. This behaviour is only achievable on the browser version of Outlook.