Magic can’t be allowed to simply happen. In a coherent (novel) world, there have to be consequences, and limits to magic add tension and conflict. So creating a magic system means the characters can’t just wish themselves happy or out of trouble. If they make a love potion, it might have a side effect of turning the recipient green or removing all their critical faculties or turning the focus of the love potion into a chocolate mouse. To be interesting, magic has to create, not resolve, problems.
And to ground the story, magic has to have rules. The reader can be amazed by the magic, but not baffled.
A number of stories deal with these issues by making magic have a price. So a wizard loses a year of life for a major spell, or the ability to dream, or has to collect magic by harvesting dragon’s teeth. But when you have an ability–say singing in tune or great hand-eye coordination–is there really a price?
So I’ve gone for the idea that magic is free. It’s an energy (for lack of a better word) available in the world, but people’s use of it is dependent on their ability to channel it. The talent for channelling is innate and takes many forms. Practice does help, but can’t create a talent. The different forms in which magic users channel magic leads to their titles of weres, witches, weatherworkers, etc. Such titles don’t define the people, but are a shorthand for their talents.