Magic comes from internal and external sources

The energy for magic comes from a lot of places. It can come from a special place (usually some spectacular natural site, like Mount St. Helens, or Old Faithful), from a focus of some kind (like Stonehenge is, on a large scale), or from inside of people. The best magic comes from the inside. Sometimes it’s just pure mental effort, raw willpower. Sometimes it’s emotions and feelings. All of them are viable tinder to be used for the proverbial fire.
I had a lot of worry to use to fuel the magic, and a lot of annoyance and one hell of a lot of stubbornness. I murmured the requisite quasi-Latin litany over the potions, over and over, feeling a kind of resistance building, just out of the range of the physical senses, but there, nonetheless. I gathered up all my worry and anger and stubbornness and threw them all at the resistance in one big ball, shaping them with the strength and tone of my words. The magic left me in a sudden wave, like a pitcher abruptly emptied out.

Storm Front Chapter 8, Page 91