The Price of Your Smile
These days smiles are precious. People tend to store them like fancy dresses and expensive suits, objects that can only be brought out on special occasions.
Maybe that's why people are attracted to Luffy. It's rare to find someone who gives out smiles are so easily, and they want to try to steal them to put away for themselves.
If she's truthful, that's partly why she joined him. She wants to hoard away the wealth of his smiles in her mind like priceless jewels.