Branwen said:
back in the days when there were very few means to commit things to memory except, well, memory itself, rural communities featuring large numbers of illiterate citizens had to come up with mind tricks in order to pass popular knowledge from generation to generation. they couldn't write stuff down because they didn't know their letters, and recording them on file just wasn't an option. so most pieces of knowledge where committed to memory by means of creating rhymes or sayings that would have made the recollection easier. it was mostly a matter of associating some mental markers to things that you could see every day, thus triggering the memory in the brain. most of these things had to do with farming and other activities closely related to the turning of the season. this is the basis for proverbs involving birds heralding such-and-such period of the year, for instance.
but of course many scraps of information were meant to be warnings to avoid getting oneself in dangerous situations, and those had to be associated with everday activities too, even though there was no sensible relation between the two. so, a black cat crossing your path would be an omen of ill-fortune
only because it could happen often and
somehow you had to be reminded to pay attention.
clearly the choice wasn't random: the colour black, and cats, for instance, have a long-lasting tradition of mysticism about them - whose reasons are a little more deep than that of popular superstitions, but let's not get into that - but the new associations do not owe much except the choice of object to the old anthropological customs.
what's interesting is seeing how most superstitions are very local: in italy, for instance, where being dark-haired is the norm, a child with natural red hair is considered - in popular culture - to be worrisome and mean. regardless of the fact that being labeled as such would probably turn
anyone into a worrisome and mean individual, the only purpose of such a belief was that of reminding the population to mistrust strangers and - to an extent and for reasons i'm sure i don't need to explain - illegitimate sons.