Speaking from complete ignorance... because I haven't started HRT yet, but I would think that the novelty has a lot to do with it. When it comes to drug highs, organisms, sky diving, or any emotionally, sensually intense experience, the first time is always the most intense, and you can never achieve that intensity again. The thrill always declines with acquaintance.
They used to believe that infants were colour blind, but a test was devised that disproved this relying on this principle. What they call "sensory boredom", or the idea that with continued, or frequent exposure to stimuli, they become all "meh", so when showing an infant a colour for awhile until they're bored, and then show them a new colour, that if they were colour blind ought to just be the same old gray, renews their interest.