Well, magic goes against the laws of the world. That's pretty much the definition of it where I come from. So when you use it, the world fights back. Usually that just means you have to put in the effort to overcome the world trying to enforce its laws, but if you're doing something really, really big, the world fights so hard to stop it that it's nearly impossible to pull it off.

I'm very powerful magically, easily the strongest Master in my war in terms of sheer magic capacity, but calling a Heroic Spirit strained even me to the breaking point without the Grail to make it easier. The only reason I did it was because... well, I had to.