Probably something in your settings for the site - you can elect which actions trigger an email notification and which don't.
If you click on the down arrow beside your name in the top right of the screen, and then select "Account Settings" from the drop down. The page you get to has "Notification settings" as a link on the right side beneath the advert box. Click on that and you should see your current options.
I suspect you have "Do not send me notifications" set at the moment. Further down you can also select which actions trigger a notification in the website and which an email to your registered account.