I believe that just as Rumpel says “All magic comes with a price”.
It really comes down to the theory that there’s always a counterpart to everything. There’s always two sides to things. You can use Yin and Yang as an explanation or actio and reactio. You can even use the physics fact that energy isn’t lost, but only transformed into something else (e.g. warmth).
If you have or do one thing it will always have an effect. It can be small, very hard to detect and easy to overlook, or it can be rather large and not always good.
If you think about the fact that magic enables you to get things that may have been out of your reach otherwise, it seems plausible that there’s always a price to pay according to what you’ve gained. Magic, in my believe is like energy. It isn’t created of nothing. It’s power comes from somewhere, may it be losing your husband (Thomas), losing your freedom (Genie) or missing to see your daughter grow up (Snow&James, I believe that is the payment for using the magic tree.). There’s always a price, no matter where the magic comes from, because essentially it is all the same, just used differently.
I hope my thoughts are understandable. If not, just ask. 😀