Of all the thoughts that have occurred to me in my life, only a handful have been truly, deeply, profoundly unnerving. Of the pair that come to mind, one I will share with you here, and the other I refuse to discuss at all, and in fact, I struggle to block it out conceptually entirely. Now that my big mouth has already caused your interest to point towards the second one, I'll at least say something about it in the abstract, and then you'll forgive me for leaving you to hang.
Just trust me; there can be thoughts that are, by their nature, terribly unproductive in the sense that one can become fixated on the thought and as a result end up suffering a qualitative diminishment of one's mental experience. In order to stay sane, we all have to fight obsession of some variety; mental modulation is a difficult skill, and it's only practiced by those who actively seek the improvement of moderation, but for those of us who still value intellectual honesty, we also have to admit that the curious mind _will_ wander, and in the interest of creative liberty, we ought not rein it in too strictly. However, some ideas can become dangerous because of the difficulty they require in order to hold tentatively, because following time and interaction with other ideas, the exposure allows a tentative idea to begin to crystalize as belief. Thoughts can affect their adjoining ideas enough to eventually metastasize into permanent beliefs in cynicism, fatalism, pessimism, and nihilism. Further, these can alter one's perception of time, space, and the nature of consciousness and/or reality. My own thought-curse affects my movement through time, and it's a mental 'skill' I wish I'd never discovered, given that our allotment here is finite. I'll leave it at that.
The lesser dangerous-to-fixate-on idea is this: as I continue to age, I can't help but feel as though all of my ideas have become repeats. Everything seems to have occurred to me before, if not in specific, at least in generalizable category. Not to brag, but I've been blessed with the curse of good memory, and if you haven't been, then let me tell you, lugging it all around gets tedious. Hearing family and old friends recite the same stories year after year—with a straight face, as though you've not heard this one before—starts to take on a less humorous and more tragic tone. Sometimes it feels like it would be nice to forget. It might also be nice to experience the moment in lower resolution: fewer comparisons, lower expectations, more apparent novelty, less disappointment.
Most seem to be unaware of their own mental degradations. The mass fail to understand how diet, exercise, and mental stimulation all play a role in keeping their minds sharp. We're too reckless and wasteful with something so precious. Is it not the natural state of things to become cold, inanimate? Heat dissipates, it moves from areas of high concentration to low, fizzling out as it spreads into a diffuse, endlessly expanded plane of near-nothingness. In contrast, the mind offers a brief spark of resistance to this cold, and its stark originality against the backdrop of what is otherwise void is something to be cherished, carefully curated, and tempered in such a way that it can prolong itself against the impossible odds.
But here's the rub: as I so cherish the novelty of uniqueness, the unfortunately necessary corollary is that I must accept that all of this exists on a curve, and ultimately all of our cleverness reduces to an extinguished ember. _Knowing_ that I am amidst this process is both terrifying and beautiful. I have watched as cleverness turned to an overestimation of itself. I have seen consciousness reduced to only a fizzle of smoke from a once flame. I've watched death slowly absorb a person in real-time, and I've accepted that this is my unavoidable fate. The terrifying part is thus self-evident to you, so long as you remain intellectually honest, but the beautiful part can be harder to see. Under the tedium of repetition, where, for someone like me, who values above all other things to travel the un-stomped path, where can I find beauty inside of this self-referential loop?
The answer is somewhat obvious, or at least, obvious enough that we don't readily accept it. The beauty _is_ the loop; if its nature is to revolve, rotate, and close back in on itself endlessly, it is the uniqueness of that strange repetitive condition that gives it its beauty. Ought we permanently fixate on linearity—what could be, what was, what will be?
Or ought we enjoy this now, as it goes around, since for us at least, it _will_ cease. Let's not allow fixations to reduce the quality of our experience. No matter how many times a meaningful thought occurs to us, no matter how many times the summer verdance quenches our eyes, no matter how many times we find the same small pride inside of a cleverly-conceived solution, the moment has none of its beauty stripped by being a repeating pattern.