Sorting Comments in a Thread -
Please see the screenshot below.
This individual view option goes along with the 'Votes' feature. It's purpose is to sort posts in the thread either by 'Votes' or by 'Date Added'. It is found at the bottom of the first post (the primary thread which begins a discussion), as well at the top of any additional page(s), should the thread get long (as is the case with the thread shown).
Clicking on either option in any thread will affect (and sort) your view in all threads - not just the thread you are currently viewing.
I suggest that unless you are interested in seeing the most popular posts in a thread, that you sort the comments/posts by 'Date Added'.