Summarized as "making a hobby your job might make you no longer enjoy your hobby".
Hobbyists photographers quickly discover this when they do a paid shoot for a wedding for example.
It turns out that running a business involves a lot more of "running a business" than the creative hobby part you thought it would involve. Doing Sales, marketing, accounting.. dealing with suppliers, employees, landlord.. etc. All the things that have nothing to do with the hobby you thought you enjoyed & were good at.