Why is the Ruy Lopez such a common opening? The Ruy Lopez opening is characterized by the moves:

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1. e4 e5
2. Nf3 Nc6
3. Bb5


Why is this opening so popular? It does not appear to be particularly special/unique for white or black, so why is it a popular opening?
The Ruy Lopez is pretty much the epitome of opening principles.  White opens with the king pawn to claim the center, and black counters in the same way.  Then white develops a knight (knights before bishops!) to attack black's pawn.  Black defends the pawn with a knight.  Finally, white develops a bishop to b5 where it works in harmony with the knight to attack e5.  White prepares to castle at the same time.

So looking at the Ruy Lopez from that perspective, any other moves would violate some opening principle.  Obviously there are plenty of openings that are played, but the Spanish Game is one of the oldest and one of the best.