Some Yahoo! services, such as Yahoo! Mail and Yahoo! Address Book, require you to periodically enter your password even though you are already signed in. We do this to protect your personal information.
You can adjust the length of time before password prompting--up to two weeks--by visiting Account Info and clicking on the Edit link for Member Information. (more)
For your security, we will always prompt you for your password when accessing sensitive personal information, like Account Info.