"It becomes a job" doesn't really satisfy me as a description for what occurs when you strike out on your own. Running your own business is much more intense and engaging than having a job. Now, it's important that you see it as multiple jobs, and not just one job. Whatever your day job is you're going to have to wear multiple hats in a startup and it's going to challenge you in different ways than a corporate job will. In a corporate engineering job, it's likely that you're working on a small subset of features of a broader product and of a broader organization. As a founder of a company, you'll be coding, promoting, designing, innovating, QA, customer service, sales, and more.
...bookkeeping, reviewing contracts, managing insurance and licensing requirements, networking, the list goes on. You can farm this stuff out when you get big enough, but in the beginning you need to rapidly learn a lot of business skills that you may have only passively come into contact with prior. You will be better for it, even if you decide to go back to corporate at some point - you'll be relieved you don't have to do all the bootstrapping, but you'll appreciate what really goes into a business operation in a way you never did before.
