Galling is pretty similar to welding, and is often called cold welding. Two like metals will often gall because they have the same physical properties. It's a lot like rubbing two candles together, rub them together and they'll stick, take them apart and you have bits of each stuck in the other.
Aluminum to aluminum, steel to steel etc will all gall.
Iron to Iron, and bronze to bronze is normally fine though. (iron has a high ratio of carbon which acts as a lube so it doesn't gall under ordinary conditions)
Dissimilar metals won't gall, but then you introduce galvanic corrosion.
Galling is not a problem for parts securely fastened together, only when they are free to move against each other.
Meant to quote wchevron