I thought the idea was that they don't owe the sufferer anything, and that's why it's tragic. The concept of flowers growing inside a living person creeps me out, so I don't look for hanahaki in general, but there is a story I like a lot where it's basically the same but a curse instead of a disease and no flowers, the sufferer will simply die eventually unless their love is requited before then. The sufferer makes sure to avoid the person they're in love with as much as possible and not tell anyone about their condition because they would rather die than have the other person be guilted into loving them.