You'll hear, "Because otherwise it's too hard." While there's some truth to that, it's hardly the real reason. The real reason is the reason that there is a lucrative industry for poetry contests: author's bias. Everyone thinks their poetry is good. Ask any startup founder to rate their idea, and they'll rank it in the top 10% of ideas. Which means 90% of us are delusional. (We'd have to be, to do what we do.)
A cofounder doesn't have to have equity or work on the project. They are the person you know who will be brutally honest with you. They have to have the ability to say, "This feature you love is stupid," and have you spend time deeply considering it. They take "you" and your author's bias (as much as possible) out of the startup.
And that's key, because if it works for you is irrelevant. It has to work for your market. No one, ESPECIALLY YOU, can say if that will be so. The best is where you can have the market itself give you feedback, but they are mostly incapable of enunciating what they truly want. So you have to guess. And when you do that, you need someone to help you winnow the chaff of what you like from the grain of what your market wants and needs.
So is there any reason this role couldn't be filled with a friend that you bounce ideas off of (assuming that friend was qualified to comment on them)?
A cofounder doesn't have to have equity or work on the project. They are the person you know who will be brutally honest with you. They have to have the ability to say, "This feature you love is stupid," and have you spend time deeply considering it. They take "you" and your author's bias (as much as possible) out of the startup.
And that's key, because if it works for you is irrelevant. It has to work for your market. No one, ESPECIALLY YOU, can say if that will be so. The best is where you can have the market itself give you feedback, but they are mostly incapable of enunciating what they truly want. So you have to guess. And when you do that, you need someone to help you winnow the chaff of what you like from the grain of what your market wants and needs.