The very important difference is that at a job you get paid regardless. You can even shift your mental model such that "it's just a job" and start leaving the crap 'at work'.

In a startup (esp if it's your own), the psychological and emotional stakes are significantly higher. Also, there's no reason that politics can't find their way into a startup you're running. You just might not notice it because you've positioned yourself at the top of the tree of monkeys.