> What are the right reasons? Most people work for money.

The right reasons for co-founding are to grow equity in the short to medium term. Obviously. To some degree you must be aware of this, and your confusion that there is such thing as "too much" for a co-founder comes across as disingenuous. These are the right reasons in the sense that align the co-founders interests with the investors. Contrast this with a co-founder who co-founds so he can draw as much investment money as possible for his salary.