Q: why does my Mac keep asking for my KeyChain password

Just while web browsing or document viewing I will get a recurring message that whatever application (mail, etc.) is requesting the Keychain password. I then put the password in and the message goes away and then in a few minutes it will come up again and sometimes one after another. Like right now while typing this note I got a -

Mail wants to use the "login" keychain.

Please enter the keychain password.

I will enter and it will go away only to return later.

Thanks.