A friend of mine asked me while we were watching Falsettos why Whizzer was the one narrating during the psychiatrist sessions. My first thought was since Whizzer has no ties with Mendel, or never taken any therapy sessions, he’s used as an outside narrator for everyone else.
But the more I thought about it, it's also likely that Whizzer's the one narrating because his presence is haunting over the therapy sessions. The first psychiatrist session we see of Marvin is about his relationship with Whizzer. Trina goes to see Mendel after learning about Whizzer and Marvin’s relationship. Jason uses his session with Mendel to discuss his fear of turning out like his father, and Whizzer is brought up again (Whizzer convincing Jason to see a psychiatrist could also be a point of contention). Whizzer's narration could be showing the audience who the main source is for the therapy sessions in the musical.
