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So, back to the point—what circumstances led to this happening?

Version one: During the Alabasta arc, the Straw Hats, Ace, and Vivi stay in a small village where a festival or religious ritual is taking place, with everyone dancing. Nami and Vivi convince Sanji (which doesn't take much convincing when his beloved melorines are asking) to put on a beautiful outfit and join the dance.

Version two: Instead of ending up on the cruise ship after running away from Germa, Sanji boards a merchant ship. Later, he's stranded on an island in the Grand Line, where a beautiful dancer takes him in. While encouraging him to cook, she also teaches him the art of dance. Meanwhile, Ace, still the captain of the Spade Pirates, visits the island and encounters this breathtaking beauty.

If you have any Ideas how this happened I very much would like to read them 😌😌

I used this as Sanji's dance reference.