Can someone tell me if it is possibly to set 'Notify me' always to on.
I want the people that comment on my website to have always a notification when someone comments their comment.
You'll have to do 2 things. First in the settings under template you'll have to require the user to submit an email (unless you are dealing with only registered users -> we'll use the e-mail that they've provided in their account).
After that you'll have to make a template override for default_form.php.