Greg hadn't considered, or planned for, the possibility of failure. Without a sink-or-swim deadline, he exhausted his savings and today is in danger of losing his home. This sort of outcome happens all the time.
Oliver and his app company came to their deadline and all of their careful planning led them to continue on with their venture. Over the past year their income has varied, but they have the right metrics setup to ensure they remain profitable and successful. They understand that it will be a while before they have consistent six-figure salaries that they did in their previous corporate jobs. So they've created an ample slush fund for the foreseeable future.
In the end, it's no accident that Oliver is succeeding where Greg failed. One entrepreneur planned in a realistic way, and the other didn't. With that being said, there's never a guarantee that a good product, good planning or hard work will make a business succeed.
The best way to improve your odds — and to limit the damage if things don't work out — is to temper your entrepreneurial romance with a healthy dose of realism.