I left my job to work on my startup full-time, and agree with all of this. It's one of the hardest things I've done. I had to detach myself from material possessions and live frugally. I don't have much of a social life anymore because it's expensive. I've had to force myself to work when I didn't feel like it days at a time. Programming was no longer fun.
If you're not ready to sacrifice everything you have, possibly including your health, then work on your project on the side. You really do have to be crazy to be an entrepreneur.
The positives: It's thickened my skin, I manage my finances better, I appreciate things more, I treat people better.