It's a generalization, but I think most women would respond this way to a scene that moved them, emotionally. One piece has quite a few of those, so maybe in the near future she's going to love Zoro, but then maybe she'll be moved by Ace's story or Sanji's, or someone else's. One piece is long enough not to stick with one character forever.
Even I, who was in love with Ace for 6 years, eventually fell in love with another charater last year.