Bringing a product to market takes a huge investment of time, energy and money. This means that if a person wanted to capitalize on your idea, they would need to stop current endeavors and refocus their life on this new task.
It is highly unlikely that the person you are sharing your idea with would want to do that. Also the type of person that would completely change their life course on a whim, probably lacks the focus necessary for executing the idea in the first place.
Most people don’t execute their great ideas. Most of the time peoples’ ideas turn out to be shit anyway.
Actually, many of ideas of successful people start out crappy as well. The difference is the successful person has the grit and drive to iterate on an idea until it becomes something great.
While I generally agree with Larsson, I would be careful about who you share your great ideas with if you live in Silicon Valley or San Francisco. Don’t get what I call, “Zuckerberg’d”.