What does "thumb down" really mean?

1) I don't like this discussion (doesn't interest me or I find it silly)

2) I don't agree with the statements

3) I don't like the person who wrote this post

4) Just kidding (or trolling)!

5) Something else ... please tell what

If 1) why not leave the discussion for those who find this interesting?

If 2) why not argue?

If 3) Get yorself a life!

If 4) This tells a lot more about YOU than about the poster!

If 5) Please tell us!

I have watched the use of thumbs up/thumbs down for a while, and I have to say that I still struggle with finding a good reason for keeping the "thumb down" choice. I don't use this option myself.