Assume a friend earns a promotion, and you want to say, "I'm happy for you."
In English, there is a difference in meaning when you say:
I am happy because of you: suggests you do things that make me happy.
I am happy for you (or on your behalf): suggests something good happened in your life, and i share your joy.
In Cantonese, it seems like the way to say I am happy for you is 當妳開心。 為你開心 would mean I'm happy because of you. The Cantonese reply to a friend receiving a promotion would be 當妳開心.
Are these Cantonese meanings and interpretations correct?
If so, is it the same for Cantonese and Mandarin, or in Mandarin does 當妳開心 carry the same meaning as 為你開心?