We see this theme preeminently in Kierkegaard and Nietzsche. But it is perhaps most colorfully expressed by Karl Rahner who described human beings as one giant decision (in his case, for or against God).
We are constituted by our decisions.
We cannot appeal to systems of law or convention or tradition as decisively furnishing instructions for life choices; every choice has to be personally appropriated.
In fact, being human sometimes involves decisions that transcend the realm of moral and conventional concerns.