Estonia! *snort*! How perfectly Snape. Just like the fact that he can build himself a new, mostly satisfactory life, with an adorable cat, and a cute little pet chicken, and a cottage I covet with all my heart, and still feel like he needs to be punished.
I love this story. I love the cat and the chicken and the cottage, of course, but I also adore Ron and Hermione--this is a Ron and Hermione I can actually envision happily together for life, which is fairly rare--and how they try to take care of Harry, however hard he makes that. Kreacher is the perfect blend of subservience and reproach. Luna is both dotty and wise, just as she should be, and her uncle Xanthus is a lovely OC (as is Eglantine).
Your magical premise is clever and well worked-out. I like the idea of magic, like most things in nature, seeking equilibrium--it just makes sense, you know?--and also the sort of non-intentionality of it, the way Harry's desire for Snape to live triggers the effect without any conscious effort.
You have a real gift for dialogue, as well; your conversations are often clever and amusing, but never feel forced or staged.
And I *love* the ending, which is just perfect, mostly, I think, because the idea that Severus trusts Harry enough to fall asleep in his presence speaks volumes about how far he--and they--have come.
A completely charming romance, and an excellent use of your prompt.