I agree with the idea of having a frank discussion with the co-founder but with respect to motivation I'm not sure it's going to be simple to get to the source of the true reason. Motivation is why I struggle with finding cofounders. I have a number of good friends who I know well, even went to school with, some of them are programmers just like I am. However they can't be cofounders because they're too happy with their lives to be motivated enough to work like I do. We have similar lives, they're just very content with the status quo, I'm not. It may be that this person could offer more shares and still get no more time out of the person, this is such a difficult problem to solve.