Please note my corrections above.
A's line doesn't make sense. We ask someone "What's the matter?" when they look sad/upset/angry etc. It would be more likely for A to simply say "Why are you so happy?" or, if you want it to be longer, "You look really happy! What's going on?"
You're lucky to have received an answer to your question without providing the source. Please note that we need the source and author of any quoted text in post #1 in future.
You're lucky to have received an answer to your question without providing the source. Please note that we need the source and author of any quoted text in post #1 in future.