I don't think you're a brat at all. I bet your T doesn't either.

Laughter covers other emotions. Possibly a slight tremor of anxiety at being offered something nice? Or a bit of vulnerability? Or a wee touch of awkwardness (the same way some people laugh off compliments) or who knows.

Before I had my own pony, a girl I didn't really know very well offered me the use of hers for Pony Club. I was so excited I reacted by giggling, and the offer fizzled out. I truly was not laughing scornfully at her pony, just from pure excitement. But we were twelve years old and neither of us had the emotional maturity to work out exactly what had gone wrong in the communication.

Your therapist has loads of training, they will 'get' it.