Hi, I'm Nick. This is my blog. I'm a life-long unschooler living in New York. You can find more about me here.
I help run the Recurse Center (YC'S10).
Follow me @nicholasbs
People get too attached to their ideas. When you think about something for a long time it becomes "yours." That makes it hard for someone else to also feel ownership over it. But if you want a true cofounder, you need someone who feels like he owns an equal portion of the idea.
That's why I think people should try to find cofounders before they develop their ideas. Even if you already have an idea, I think you're better off throwing it out and then looking for a cofounder.
At a minimum you shouldn't make working on your current idea a prerequisite for someone being your cofounder. If you do, you're taking a really hard problem -- finding someone you can start a company with -- and replacing it with an even harder problem -- finding someone you can start a specific company with. That's probably an order of magnitude harder to do.
Look for a cofounder before you seriously develop an idea, or at least keep yourself open to completely different ideas. Find someone you gel with, and then develop an idea together.