On the flip side, working for yourself means the job never ends. As an employee you can do 8 hours at work, 16 hours for yourself. You can drop a project and pick it up the next day. The client is pissed? You're still making your salary. It's up to management to staff correctly, not up to you to kill yourself overworking for a salary and no overtime.
When it's just you, and your entire livelihood depends on sucking up to big fish to get that one payday that will float you for the next few months while you find another big fish, the boundaries between work and home dissolve. You feel a constant stress to get projects done and you even lay awake at night terrified of what happens if your next big fish doesn't materialize. Your performance in any one task decreases because you have to wear every hat. You're a good programmer, but are you a good businessman? Advertiser? Hustler? Lawyer? Can you afford a team? Will do you everything yourself?
If you're still a kid, don't have debts, don't own property, don't have a family, don't have any roots, the risk/reward and excitement of this chaotic and generally financially ruinous lifestyle could be good or even make you happy.
But for many people, the stress of putting your family, your investments, your retirement and ultimately your future on the line is one of the most unhappy things possible.
I don't think there is anything wrong with doing your 8 hours to make a living, and then taking the next 8 hours to enrich your life -- not for profit, but for health. Exercise, learning and growing, cooking and entertaining, starting a family, investing in life and happiness outside of coding... it's just notoriously difficult to do these things as an entrepreneur, and all the "big successes" as a rule (with exceptions) are "16 hour workers", as in, 100% of useful waking hours spent on the job.