I'm not sure what kind of "puzzle" you may be referring to. It usually pretty obvious from a big picture standpoint why a patient is in the unit - can't breathe, pressure sucks, septic, etc. The specifics of which are mostly tedious details that most of the time have nothing whatsoever to do with critical care in the least . . . "Oh this patient has cardiogenic shock from STEMI, with a balloon pump, on pressor for a likely sepsis they got from a pneumonia vs uti vs line infection, now going into ARDS and renal failure, on CRRT, with a gap and non-gap acidosis from all the saline and elevated lactate." Where's the puzzle and wonderment? Keep the patient alive, and keeping the patient alive IS critical care, and one of the reasons midlevels can do it. There's almost never anything magical or complex about the diagnosis, and even if there was, treatment is largely the same.
I work in a closed uint, and I take patients that were dead and make them alive. I try and figure out why, but it's not magic or mysterious. The reasons people get sick enough to get stuck in an ICU is largely boring and mundane. Even the transplant patients with their weird bugs, immunosuppression, and outrageously deranged lab values don't change much of what is done in an ICU. Though we did see an HLH last week which was the most interesting thing I've seen in a long time in the unit - ID pretty much shat bricks when we took the patient off of abx and started steroids.
Saving lives, curing dz, and otherwise simply living the dream.