Three possibilities come to mind:
Is there an evolutionary purpose?
Does it arise as a consequence of our mental activities, a sort of side effect of our thinking?
Is it given a priori (something we have to think in order to think at all)?
EDIT: Thanks for all the responses! Just one thing I saw come up a few times I’d like to address: a lot of people are asking ‘Why assume this?’ The answer is: it’s purely rhetorical! That said, I’m happy with a well thought-out ‘I dispute the premiss’ answer.
Behave is incredible.
I read a fucking lot of books about what makes us tick. Behave is (tied for) my favorite. He does actually hint down the determinism path a little in behave, but he goes all in on Determined.
I would still probably generally recommend Behave over Determined, but Determined is directly relevant to the OP.