The only rational conversation to have is that people tend to be influenced by each other. This is nothing new or shocking. If you liked something, but you went with 5 friends and they all hated it and spent the entire night talking about how bad it was and all the things wrong with it, it is possible, even likely that your opinion might be lowered. On the other hand if you somewhat disliked it, in the same scenario you would probably end up outright hating it too. Flip it around, if everyone in the group loved it, appreciation for it increases as each person emphasizes the parts they liked most. If you disliked it, in such a scenario you might see the positive side and be less harsh towards it. This can apply to anything - film, music, food, literature, games, etc.
Why is it that this "conversation" only comes up when something is being negatively trashed, and not when there's an overwhelming unanimous positive reaction for something? Because when something is being trashed, someone gets hurt and upset, and wants to rationalize it. Again, that's also human nature. Not sure if it's really that interesting a conversation.