I like the classic trope of dragons loving gold and collecting it in their lair, yet it seems poorly justified. I know that it is often used as a metaphor for greed; "dragons disease" to show how material wealth corrupts people. I would like the same behavior in my dragons, but with a realistic and scientifically plausible explanation.
Dragons in my setting are a bit different than classic dragons. Think of a Quetzalcoatlus, somewhat larger (up to 50%), wider wings, shorter neck, more dragon-like head, a longer tail, strong regeneration ability (common for monsters in the setting as it is a post-apocalyptic-modern-fantasy setting and without it, an AK-74 will counter most big monsters), ill-tempered and a living napalm thrower. They are quite clever, but nowhere near human-level intelligence. Males are less aggressive, smaller and roam the countryside. Females are very territorial and have lairs, a nest and the surrounding regions which are death-zones. They use a quantity-based reproduction system. The female lays dozens of eggs and the small dragons are left alone to fend for themselves, resulting in them hunting down everything they can find and even resorting to cannibalism to sustain their fast growth. The few survivors then continue to develop wings and usually leave the area.
So why would these dragon lairs usually contain big collections of shiny stuff? Note that shiny stuff does not necessarily mean gold, but what we would call a garbage dump. Copper pipes, car parts, aluminum foil, knives. As long as it is shiny it can be found in a dragon lair.
Now dragons did not evolve in our world (the setting is actually our world) but were transported here by the same event which brought magic into our world. So why do dragons, which evolved in a wild world with nothing but untouched nature, start collecting shiny garbage dumps in their lairs once they come into our world?