I find humor in therapy a very difficult thing. But maybe that is because I like using sarcasm... And for one or another reason that's often not the best thing to use with a therapist (in my experience at least). Quite a few times it turned out not to be funny at all and instead left my T wondering why I said the things I said. But how do you then tell your T that he/she's probably not getting your kind of humor?
So, for this reason, I don't mind the fact that my sessions stay 'on topic' most of the time and consist mostly out of rather serious conversations.