At some level, I’m going to go with “psychological quirk” theory, except that it’s more common than that, so maybe not exactly a “quirk”. After all, so many peoples’ need for religion is so strong that if they didn’t have it, they’d have to invent it. Which quite probably is exactly why we have religion.
It’s a nearly-uniquely human quirk that we have a strong ability to associate symbols with real-world objects (nouns), actions (verbs), and attributes (adjectives, adverbs). That is exactly what our use of language is all about. This works for words (spoken), visual representation (written words), and all manner of other signs and marks (symbols). We associate the symbols with the things they stand for.
Sometimes, and more so for some people than others, there is a tendency to confuse the symbols with the things they actually stand for, and react to the symbol as we would react to the actual thing. The more a symbol is associated with a thing, and the stronger reaction we have to a thing, then the stronger reaction we have to the symbol as well. Clear modern example in Western thought: Our reaction to the swastika (which in fact has been in the news and on this board lately).
Religions are heavily inbued with symbols (some more so than others). The Catholic religion, ISTM, is very heavily into symbolism. Other Christian churches also to varying degrees, and all other religions too, to whatever extent. And there is a tendency to see your own symbols as very good, and others’ symbols as evil. Witness the chronic ranting about the Proctor & Gamble “man in the moon” logo, for example. The more fundamentalist-thinking people are the most likely to do this.
Political ideologues also commonly think like this. In the days of the anti-Vietnam-war peace movements, for example, the “peace symbol” was seen by right-wing political ideologues as a Communist symbol, the very presence and sight of which would capture the minds of our youth and brainwash them towards the ends of the gleeful godless Communists (well, the fluoridated water helped too).
There is evidence that other animals do this too, to a lesser extent. There have been all those animal-language experiments that taught some rudimentary language skills to chimps, gorillas, dolphins, sea lions, and parrots. There was one study I heard of with raccoons, who instinctively wash their food in rivers before eating. Some raccoons were taught to accept tokens for certain behaviors, which could later be exchanged for food. When they got the food, they washed it. But after a while, they began washing their tokens too. This is a non-human example of animals starting to treat their symbols like the real thing that those symbols stand for. (ETA: This came from a book called “How Animals Misbehave”, IIRC, but I don’t know the author off-hand. Maybe I can find it Real Soon Now.)