Because it killed me when Luna cried to Gentiana how much she wants to listen to Noctis's voice again. The last time she heard Noctis he was 8 years old child, so the whole scene gave me a very, very unsettling vibe.

Let Gentiana (or Ravus? whoever you want, really) make Luna realize she idolizes a pictures of a child she met once. It could be cute when she was 12, but now she's over 20.

++++ bonus points for someone pointing out to Noct "you know she's not the same 12yo girl you once knew, right?"