[kalavan]

It's considered mysterious because of the hard problem of consciousness. Describing a mechanism that could be considered analogous to consciousness "from the outside" is pretty easy (just do self reference).

But the subjective quality of "what is it like to be X" is not easily captured by such descriptions - not unless you make some kind of panpsychist assumption that everything that has self-reference is subjectively conscious.

That said, a number of materialists say that there's no there there, and thus no problem. We're just all deluding ourselves into thinking that we have subjective experience. I don't think that argument is very strong, but it is made, and could explain why some find the whole business of consciousness seemingly trivial while others consider the hard problem to be very hard.

More information can be found at https://iep.utm.edu/hard-problem-of-conciousness/