If they don't like your opinion, or sometimes I've noticed it's because they personally don't like you. I've gotten down voted on the most innocent, non-controversial opinions. It's their passive aggressive way to get back at you while not actually having to talk it out, cuz it's too hard for basement dwellers.
Not liking your opinion, or not personally liking you. Personally, I couldn't give two shits about down votes or upvotes. I mean, am I gaining points or losing points if I'm being up voted or downvoted? No. You shouldn't feel that way either.
Now if we can see who up voted or downvoted you, then it would mean a lot more. However, G@G knows that this is a recipe for disaster so they won't take that chance. Haha.
They likely just don't like you or they can't pick up on the humor. I had my most basic comments downvoted to hell for no apparent reason other than people don't like me or they don't get that I'm joking.
I think the return of the downvote has made some users downvote crazy. They missed out on being able to use that feature for months, so they go around and downvote whatever. There are people that do it just because (though I don't get that), because they dislike you, or because they don't like whatever you have had to say.
For awhile, before I set my profile to private (mainly because of how many weirdos were on it), I had a disgruntled girl who didn't like my answer to her question and who had about 9 fake accounts go around disliking my answers with all of them. The most humorous part was when she used all of them to reply "I agree" or "Couldn't have said it better" to her rude comments on my opinions. Hmm... I don't think it's a coincidence that I kept getting 9 down votes, there were 9 accounts bashing me who all had similar names and were either 29 or 30, and all had the same typing style.
Some people simply down vote out of a difference of opinion, even if it's something small.
Then there are people who down vote because the opinion is rude, unhelpful, or gives bad advice. This is what I use it for.
Generally if they say something stupid like "No, virgin hunter, you didn't molest that girl. You got her drunk so she would make out with you, and you took her far away from where she lived, but it's not your fault you touched her in ways she didn't like or want. You are blameless in this situation. Women are such liars."
Most of the time they disagree with your opinion or feel like you're being offensive. There are also people who do it just because they personally don't like you. You can say something like 'the sky is blue, cars have wheels' and still get downvoted.