I experienced the same thing and it made me uncomfortable, like I should be happy but I was upset. It kind of challenged me because I was owning my negative feelings, then she gets happy and I feel like I need to be happy too. This makes me feel like she's forcing me to be happy (although she's not).

On the other side, if I were a therapist and someone made some improvement I imagine I would be happy, especially if it was something they've been struggling with for a long time.