Whenever you have two people, objects or ideals clashing or working together, what better way to symbolize the connection with the elements.
Since the dawn of man. fire
were used to symbolize events, ideals, traits and over time, foils and rivalries. So placing two of these elements together would make good visual contrast.
Compare Elemental Rivalry
which has some overlap
Okay I'm been meaning to get to this since Lightning/Fire Juxtaposition
was launched, but I forgot (and was busy with another YKTTW
.) I already have two ideas for the internal subtropes: Air-Lightning Juxtaposition, and Earth-Air Juxtaposition. But I'll get to work on those sections later.