I do have a theory on that matter, that goes a little bit into the metaphysical works of the brain.
First of all, it really depends why you are using it. Most smokers do it out of habit, but not for special reasons.
I use it to achive a certain level of creativity, relaxation and mind-drift when I am working on some stuff. I love to utelise it in conversations.
I don't use it just for getting wasted. And I really don't need a lot either. My consumption varies a lot. From a couple of small joints, to a big one, over many days or weeks without any of it.
When I was younger, I used to smoke quiet a lot. Until two things happened.
I realised, that the first thing in the morning was a joint. For no apparent reason.
And I had a spontanous pneumo thorax (by accident) without noticing it for a long time - what happens is, that air flows into the space between your lungs and you chest. Usually there is a vaccum, that makes you inhale (we inhale passivly and exhale actively by compressing the phrenic - solar plaxus area).
Since I did not know, it really felt weird when I was stoned. I had several very hard panic attacks while being stoned and these bad trips still haunt me after 10 years - there are still occasions when my lung area feels weird - wich is of course subconcesly accociated with thos bad trips.
So I stopped for some years and it took me about 5 years until I could enjoy a hit or two - thanks to my aussie-friends, which helped me alot in '99, while I was living in Aire, picking Pumpkins and Watermellons.
My personal theory has to do with what THC does. As mentioned it opens your mind. But to what extend? I think that we walk through life and collect information, that is processed in our brain. Up to an amount, we are capable of, depending on where we stand in our personal developement and evolution.
After 20 years of self-exploration one starts to be aware of most things anyway. You don't need any substance to help you process information in an prior unusual way, because you allready do it.
So the amount of information-processing, that makes use of THCs power is reducing.
But if you stop for a week, or two, you will have collected a little pile, which might again use the effects of THC, thus giving you the known experience.
If you smoke every day, you are so used to process the small amount of information, that you don't realise it. But it is still there...
You can try by stimulating your brain. Do stuff, you usually don't. Talk a walk, read a difficult book, do some arts, whatever might trigger the information collection and raises the amount of it...
Then you will see, that the effect is still there, but you just didn't noticed it, because you are used to the low-level-processing allready.
And then of course the sort of weed is important
Try switching a little bit...