Hawk may be feeling a little deflated at not getting any response to this question yet (everyone give her big hugs ) but I think I understand why. Knowing your "why" is a) very difficult to identify and articulate, so can take a long time, and b) often quite personal.
Luke and I were helping to facilitate a workshop today, actually, and one of the activities for the participants was to write down the purpose of their business. We asked a couple of people if they were comfortable sharing, and some did, but others preferred to keep it to themselves, which is totally fine and understandable.
I will say this though: if you can identify and articulate your "why", it can be extraordinarily helpful when it comes to making decisions down the track. Understanding your purpose means that you can ask things like: "Is saying yes to this particular opportunity in alignment with my purpose?"
So yeah, if anyone wants to share, awesome. And if not, completely cool. But you should still go through the difficult task of writing it down—it's for you, after all!