I believe that the main problem is this:
Loudly declaring that you LIKE something is seen as positive and life-affirming, and lifts everyone up.
However, loudly declaring that you HATE something is seen as an insult and implicit threat to all those who like the thing that you just said you hate. This is because hatred is inherently poisonous and violent.
And about being shouted down: If those who actively control (moderate) a forum, or even the entire board, have an opinion that you do not, that is a very real problem...
But publicly declaring that you hate something should not be taken personally and that is where the problem lies.
ie: I hate this certain something that you like. That doesn't mean that I hate you. Yet it is taken personally, so the issue is not with the person hating the item, but with the person liking it. They can not separate themselves from the item they like so by you hating the item, they feel you are hating them as well...and so they become defensive.