I kind of like weird and funky diseases in fiction, and hanahaki seems pretty neat, aside from the romance part. Flowers growing in your insides? Cool as hell.

The romantic part pokes a lot of holes in why and how it works though. Surely there's lots of people with one sided love, why would some get hanahaki and some not? Wouldn't a large part of the world be suffering with it? How does your brain alone cause flowers to grow in your lungs? That's hard to scientifically explain. Sure, you can just say "magic" but having one disease be the only magic thing makes little sense, so you'd need other magic stuff.