I won first place in the writing contest.
- I'm so happy for you.
(In this dialogue, what is the meaning of 'for'? What else can we use instead of 'for'?)
I don't know of anything that could substitute for 'for' here. It's something of an idiom, meaning, 'Congratulations.'
Is it "I'm so happy because of you."
That has a completely different meaning.
"I'm happy for you" is sending congratulations and happiness TO the "you."
"I'm happy because of you" focuses the happiness on the "I" --which is the opposite of what you posted first.
The word "for" in this context is used to convey the feeling of happiness or joy on behalf of someone else. It shows that the person speaking is expressing their happiness on the victory of the writer.
In this particular sentence, "for" can be replaced with other prepositions like "about" or "over" to convey a similar meaning. For example: "I'm so happy about your first place in the writing contest" or "I'm so happy over your first place in the writing contest". Both of these alternatives express the same feeling of excitement and joy on the writer's achievement.