Ah, to have that permanent state of bliss. Don't you want it?
We want that unshakable assurance that we know. The kind of assurance that can't be questioned because the authority of it is so profound, and the experience so subjectively felt, that we cannot doubt it. An experience that is so far beyond logic, that no argument could even begin to lap at its feet.
We may be programmed to want this - to search for this. We don't get this desire from reading books. However, books or a teacher may make us aware that such a desire was already nascent in us.
A touch of spiritual insight just uncovers that desire. Then the search is on! We don't get this desire for profound assurance from reading. It is our true nature. Our true Self wants this. Our true nature is sure. Somehow, that allness wants to experience itself through through us.
As for such high experiences coming and going? It appears to be a normal progression. Cyclical, like most of nature.
There are gurus who express what they know from an assurance of permanence. Mooji often says "I don't understand how you say you got it, and then..., and then.... What is this and then?" Obviously Mooji is implying, if you get it, it should be permanent.
My own experience is that I have had the complete and total experience of oneness and meaning so profound that all normal feeling and experience is meaningless in comparison. And yet that state, experiences of that state, don't remain permanent in this body/mind.
This doesn't mean that I am not changed radically from having had such experiences. I had to let these states go because I just couldn't function in this world and remain in such states. More likely it wasn't a choice. I just couldn't stay there. And I couldn't stay there because there was no "me" there.
Would I like to be able to stay in a state of bliss, without fear, in total oneness, and assurance that it was permanent? Of Course I would. Could I? Who knows?
What helped me a lot with this question of states of bliss and oneness and their lack of permanence was reading "The Experience of No Self," by Bernadette Roberts. She describes high states that come and go. Then even higher states that come and go. Her point is that once one is in a new experiential state for awhile, one gets used to it, so it is no longer experienced as a "high." It becomes the norm.
The bottom line is to know what you know, but remain in the unknowing of what you don't know, even acknowledging that you don't know what you don't know.
If extremely high states are not permanent, then there is more work to do, or such a permanent state is not in the cards for you. It's the journey that matters.
Our intellect does want as much understanding of the mystical as possible. Even though spiritual experience is beyond the intellect and logic, the intellect does want as much understanding as possible.
It seeks a level of satisfaction. If it can get a level of understanding, it actually relaxes. And that relaxation may allow the spiritual seeker to really let go, since with enough feeding, the intellect sees its own limits and accepts them.
Permanence and impermanence are in a dance. We are the dancers. Enjoy the dance.