There are some stalwart theories as to why people love a mystery. Not thrillers, mind, where the excitement or suspense is the central trick, but mysteries. Detective stories, cozies, whodunnits.
For many years, the theory has been that people like solving puzzles. They enjoy watching a character find the solution, and the challenge of trying to find it themselves. But I have never been a gamer or a puzzle fan. Sure, I do the occasional crossword, but more for the words than for solving it. (Years ago when I played Assassin’s Creed, I only did it to see the scenes of Italy and London.)

Another theory is about justice. Humans like seeing justice done. Whether that justice is legal or divine, we like seeing the evildoer get his or her comeuppance. Certainly I want humans who do bad things removed from situations where they can continue doing them. I strongly believe in societal systems of justice, and enjoy studying them.
But it occurred to me that there’s another reason we like the whodunnits. In most mysteries, though not all, the solver of the puzzle is motivated by helping people. While the murder victim may be dead, the living have questions, pain, and a need for resolution. If you’re a suspect, and you didn’t do it, you need help even more. The detective, whether professional or amateur, uses skills not common to everyone to find the miscreant, and often those skills are appreciated, at last if not at first.
It doesn’t matter that sometimes the detective doesn’t seem to care about anyone. I’d put Lord Peter Wimsey in this category, and Sherlock Holmes. Even Miss Marple keeps herself removed from the angst of a case. Others, like Benoit Blanc or Precious Ramotswe, seem to care a lot and become involved in people’s lives.
Those who get emotionally involved are often appreciated by those they assist. The detectives who distance themselves get satisfaction from what they do, regardless of whether they are thanked or not. We, the readers, provide that appreciation even when others do not, making us complicit in their helpfulness.
I might add the caveat that remote characters such as ol’ Sherlock Holmes run the risk of never evolving very far because of the distances they keep. He is seldom a changed man as a result of what he faces, even after kicking cocaine addiction or killing an opponent at a waterfall.
Alternatively, someone like a Benoit Blanc seems to be effected by his cases: he’s more self-absorbed and egocentric after encountering the mega-influential of “Glass Onion”, which carries over into the latest, requiring his “road to Damascus” self-realization. The character has a personal arc that changes him somewhat with each encounter. His growth is part of the narrative.
I guess it depends on how infallible a protagonist the reader responds to best.
Very true. I think I rather prefer the stability of the protagonist, if not his/her infallibility. Although I do recall in the later episodes of Poirot, the writer David Suchet doing a character arc even though it’s not really there in Christie. I rather liked it.
Exactly!