Most of the people just leave their jobs because they only want to give more times to side jobs and these side jobs becomes their permanent jobs later for example most of the freelancers leave their jobs after they succeed in freelancing and giving full time to freelancing.
Many of my friends also do this, but the competition to become a freelancer is very tight, so leaving the main job to focus on a side job is very risky. As long as your side job doesn't interfere with your main job, why should you leave your main job? just do both...