Head Canon: To grow, dragons need to anneal themselves in something molten… hence his love of “seven hour bubble baths.” Most dragons do this in lava, those a privileged few will find molten minerals alongside the lava. If Spike were to go with Twilight on an inspection of the Equestrian Treasury Forge and witness gold nuggets being melted down for purity he’d find it hard to resist the instinctual urge to jump in. Since gold/platinum is so valuable he’d emerge as something akin to a god.