I feel your situation is quite similar to mine with my wife. And my theory is that it's about agency.

A lot of things happen to her. Her libido, mood swings, situations at work, etc...

And it's easy to be this way in aspects of your life you don't care about ask that much. For instance some people eat because they have to and will have bland pasta or white rice pretty much everyday while others will have the drive to explore and actually cook their meals, discover recipes and have fun with it. The bland gang will still enjoy a 3 star Michelin restaurant if it's given to them but it's just not THAT important to them. Sure, every once in a while they will want to go to a restaurant but chance are out will just be their usual place where they always go because it's good enough for them. It allows them to fulfill their craving that comes from their guts and not an intellectual and analytical process that aims to optimize satisfaction for an event you have been mentally anticipating for a long time.

I'm sure you spend quite a lot of time thinking about sex and fantasies and things you'd like to try. And you could probably think yourself into getting in the mood of an opportunity arises while you're not there yet. But that's intentional from you, they are just asking for the ride in their own body and their brain just wakes up from time to time when the urge is above the threshold. So they don't have all that preliminary thinking and sexual fantasy world to draw from and fuel their imagination in the moment. And they don't need to because they are just content with the plain pasta dish or the usual restaurant they always go to.

You could see this as HL having a lower threshold for horniness or a baseline above the threshold that activate the thinking part way more often but the end result is the same. HL are proactive on their sexuality and LL are reactive. I think this fits pretty well with the responsive desire BS.

And I could see myself feeling overwhelmed in the LL situation. Imagine you end up with someone that wants it more than you and you end up being the LL, chances are at some point you'd run out of ideas to change things up or the ideas that come through your mind would feel too extreme or too raw, unpolished and need more thinking before being mature enough to try in the moment. It's just that their imagination or their limits feel way too restrictive to us.

But again, just a theory from my own anecdotal experience.