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Did someone post this theory for Serena dance episode?
Since it's required Serena dances with a male, I think Serena chose Serena because we all know how bad Ash is as a dancer as seen during Corni's battle and episodes where he tries dancing.
I seriously think this is why Serena is dancing with Clemont now instead of Ash and Millefui had no males to dance with so she insisted she had to dance with Ash which Ash ultimately agrees upon.