This term originates from Facebook, where you can publicly say you "like" something, and then have the option to take the "like" back. "Unlike" does not mean you dislike something.
To put it mathematically:
If Like(7)=14, then Unlike(14)=7
but if Unlike(12)=5, then Like(5) does not necessarily equal 12.
I thought I would buy that new dress, but when I saw the fabric up close I unliked it.