If people dislike a thread, they put a reason why in the comments. There is always a reason why they dislike it. If this was added, there would be more people just pressing dislike and not giving feedback on the thread.
Also, let's say 2 people fell out. They might dislike-spam other people.
Finally, some people may get upset that someone disliked their post, so it would cause negative feelings.
Sorry, but -1
(If this was here, I'm sure many people would just dislike it and not post why they don't like it ;))