Also, under what circumstances exactly do resonance structures occur? I'm curious.
Will they always occur if there's a negative one formal charge and a double bond nearby?
Resonance structures are a lie. These are the results of using simple Lewis structures to explain "intermediate" bonds.
The fact is, bonds are never strictly pairs of electrons. The electron density is instead smeared over the molecule, though most of its density is localised between two atoms, like a bond. In the case of "resonance structures", these are ways to represent groups of electron that don't "belong" to a particular bond, but rather is smeared over a larger group of molecules (e.g. in benzene, 6 electrons in the ring are smeared over the ring, rather than belonging to any C-C bond). A much better representation would be to show these delocalised electrons as they are.
These delocalised electrons are often represented in Lewis structures as alternating double/single bonds, usually called "conjugation". Sometimes (especially in hydrocarbons), delocalised orbitals do not necessarily mean there are multiple resonant Lewis structures (e.g. hexan-1,3,5-triene has a certain degree of delocalisation, but no resonant structures).
tl;dr, resonant structures are a lie.
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