There is always a simple answer to such question, and it's usually about some inconvenience the service provider decided to set-up for the user. In this particular case I think the answer is obvious: email provider usually have a session which never really ends, and just sits there logged in unless the browser cache is wiped.
Make your service auth token to live for the same time as Gmail's, and as an alternative give users an ability to just login with OTP every time, but stop these unholy 12 hrs time-to-live auth token practices - your users will never log-in via password restore again.