From your experience, I gather that not only does transition resemble a rebirth or 'second puberty' to some trans people, but their euphoria can be from a feeling of escapism from their old life if people end up treating them very differently afterwards. That's what individuals in a life rut/crisis tend to relish if they are seeking to shake their life up. That and an(other) opportunity to transgress social acceptability/norms. It's almost adolescent in its attitude. Maybe that's also why they so often model their behavior after how they think youth of the opposite sex behave. It sounds like that person you know has managed to sate that hunger for escapism and change so is satisfied the way they are now, and is settling down.