Near the bottom of this page, there is a little bell, which says that it is either Muted, Normal, Tracking, or Watching. You can change it to Watching if you want. Do you see that bell? What is it set for now?
The bell is below the row of buttons labeled:
wrench-picture, Share, Boookmark, Flag, and Reply.
Further to this; for topics that you started or have responded to you will automatically get a response without having to set the watching flag. See your default settings in your profile → preferences → Emails Under there you will see the general option:
Email me when I am quoted, replied to, my @username is mentioned, or when there is new activity in my watched categories, tags or topics
This is applies to all topics you have started or responded to and by default is set to “only when I am away” ie. only emails if the system hasn’t see you logged in for 10minutes.
For all other topics that you are not involved in you have to do as Patrick says and set the watching flag.