Just because you made some piece of software doesn't make you an expert. Just because you can ride your mountain bike over those rocks doesn't mean you're a professional. Just because you can huck those cliffs on your skis won't make you the next Lindsey Vonn. Just because you spoke at a conference doesn't make you the master of the universe. Just because you blog doesn't mean you're a good writer.
So what makes an expert? Not sure there is a good one, but I saw this quote by American writer Denis Waitly and thought it the closest we'll ever get: Never become so much of an expert that you stop gaining experience. View life as a continuous learning experience.