Guy encounters girl for the first time at a bar. She is sitting alone guy approaches.
Guy: Say, that's a nice blouse you're wearing. Girl: I don't like it. It's not my color. (Looks away.)
Now, you experts in Western culture tell me what is going on here. Can you translate for me what the girl is really saying?
Amplify such nuances by a hundred fold and you'll get something considerablly less dangerous than the Japanese language.
Consider the dialog below:
Girl: I loved that movie "Bridges of Madison County". Guy: You couldn't pay me to watch that movie.
In English, the guy is expressing his thoughts on (1) the movie, (2) the category of the movie, (3) his own tastes and (4) his own temperment. In Japanese, he would also be expressing (5) his liking of the topic of conversation at hand, (6) his evaluation of the girl's tastes, (7) his evaluation of the category of people the girl belongs to, (8) the placing of said category in his estimation, (9) the direction that his mood has taken due to the subject of conversation just introduced, (10) the liklihood that he would continue with this topic of conversation and (11) the overall station the Girl occupies in humanity. If the guy had been smiling while he said it, the gravity is greater.
Those are just the ones I can think of off the hat.
But this is only standard Japanese.
If this was in, say, Kyoto (a.k.a. Assholeville) you may have the following conversation. (The Japanese words are in red.)
Guy: Fine robes you are wearing. ええべべ着てはりますな Girl: Why don't you step in for a cup of tea? あんた、中に入って茶でも飲みはりますか
Wanna guess what this means?
I never bothered to learn Kyoto dialect properly so the above lines may sound a little less than authentic. But my wife went to college in Kyoto and she has told me some horror stories.
The above example can be translated to mean something like this.
Guy: What kind of an insane clown wears an outfit like that? Girl Why don't you get off my property before I call the cops?
All of this doesn't mean that you cannot disagree with each other. (The guy and girl in Kyoto in the above example clearly disagree with each other.) What you must avoid is the act of being rude by accident. You can disagree about movies without making judgements on her character. Especially if you are good friends. Just make sure that you don't sound like you are.
It amazes me no end that every American who has steped on Japanese soil since Commodore Perry onward has noted how polite the Japanese are, yet to this very day Americans have not yet figured out that they are often very rude by Japanese standards. It's the two sides of the same coin. It should be obvious. But foreigners are always bewildered at the reactions of the Japanese people to their well-meaning or neutral remarks. It never siezes to amaze me.
Next time she recoils to a simple disagreement like "I don't like that movie", try to realize that you just did a no-no. There is nothing wrong with her culture or her upbringing or even her assertiveness. It's you who just screwed up.