[ There are several answers she could give to this. The truest is also the hardest to hear, and she doesn't want to scare him off completely. But any other, lesser reason might not be acceptable. ]

My home is here, now. I have so many friends from so many different worlds... I have so many memories from this place. I want to stay because I don't want to give all of that up. I have a life here, Sanji.

That doesn't mean everyone will, or thinks the same way I do! Most people who are here want to leave, so you're definitely not alone in that.