You will need to modify a Velocity file to achieve this:

  1. Open <confluence install>/confluence/template/includes/macros.vm and look for comment.notification.enable.
  2. Replace value='false' to value='true'.

It should look like this:

	#if ($remoteUser && !$action.userWatchingPage && !$action.userWatchingSpace)
		#bodytag ("Checkbox" "name='watchPageAfterComment'" "theme='notable'" "label='comment.notification.enable'" "value='false'")
			#param ("tabindex" "203")
		#end
	#end

A user will automatically be assigned as a watcher of a page after they have posted a comment on the page. Even if their comment is removed they still will receive notifications if there is an update made on the page. To stop this, they will need to remove themselves from the watcher list.