There is a predisposition in human brain for unsubstantiated beliefs, more in some than in others, and apparently genetically inheritable.
I am as atheistic as they come - never in my life having the least doubt about it.
Yet I found myself strangely uncomfortable faced with common superstitions - like spilling salt, breaking a mirror, having a black cat cross my path, etc.. I have never ever believed in such stuff and yet felt strong urge to act upon them. I didn't, nothing bad happened yet the urge remained - for decades.
Relatively recently I figured out that there is just some unconscious part of my brain that keeps track of those things and fires a (distracting) emotion of discomfort for a short while. A sub-personality, if you will, that probably serves some vital functions besides making nonsense. Maybe it has something to do with obsessive behaviors, of which Tourette's is an extreme.
Armed with that knowledge, I can now perform the "ritual" (knock on the wood, etc.) - not afraid to slide down the path of irrationality, but merely to throw a bone to that pesky sub-personality and shut down the emotion, rather than have it distract me and potentially interrupt my train of thought. Like scratching an itch.
Now, if someone as skeptical and open-minded as I am has issues with emotional discomfort, I imagine in some people it can just be unbearable...
Another thing, besides the superstition about a broken mirror - about which I heard from my parents, though they did not "observe" it, everything else I - or rather, the sub-personality, learned from other children or books.