Passion without genuine skill and knowledge is useless. You can be passionate about many things, for example cars, but unless you put in the hours, your passion doesn't mean you will be a great car engineer, or automotive company executive.

[EDIT] Also want to add that people grossly underestimate the responsibility of being their own boss. There are a lot of tasks you have to handle outside of your usual work like finances, scheduling, client outreach, etc...

The OP seemed to understand this, but a lot of people will create a start-up for the sake of being on the creating end of something without really knowing how practical their venture really is.

Paul Grahm may disagree though.