This question has been raised on https://security.stackexchange.com/questions/40723/why-do-you-need-to-login-after-you-change-a-password - With security in mind
As one user states: "From a security point of view, whether the "password reset" logs you in automatically or not is rather neutral."
So why do we enforce the user to re-type the same email and password just provided by the user?
Email -> Unique token -> Set a new password -> Redirect to login page -> Type in email again -> Type in password again -> Log in user
Email -> Unique token -> Set a new password -> Log in user