@i-am-insane I think for a lot of us there might be an important distinction to make here. Are you asking us why we find the fantasy appealing? Or are you asking why a a character in a story might want to actually shrink?
Because for me, I could tell you all the reasons I love the idea of being tiny, but a lot of it works much much better in fantasy than it works in reality. The fluff stuff might transfer over (like the idea of being comforted and protected and cared for by a giant, that sounds pretty good IRL). I could go on and on about that if you like~
But a big part of my kink side is being dominated - someone else making me smaller and smaller and having their fun with me. The loss of control and the fear itself play a big role for me in that fantasy - but of course, that doesn’t mean I want to be violated in real life. So for a character in a story, I’m not sure that kind of motivation would necessarily work? I suppose you could have a character who has a size kink and so is curious to explore that if she gets the opportunity (and either it’s all sweet and consensual, or she gets in over her head). But if you’re wanting to write a character who doesn’t have the size fixation to begin with but still wants to get small in the story, that might affect her motivations.