Desire is trying to obtain something that you think will make you complete — or at least, more complete than you are now.
Fear is hanging onto and trying not to lose something that you think makes you complete — or at least, more complete than without it.
Self-realization is the only thing that makes you truly complete, and thus a desire for enlightenment is different from all other desires. And yet even this desire must be exhausted before self-realization can become even remotely possible, because self-realization means acceptance of absolutely everything as it comes to you in each moment, while a desire — even of enlightenment — implies dissatisfaction with the current state of things.
The Witness, then, is what remains when the desire for self-realization has burnt up in its own fire, leaving behind just the state of peaceful awareness. After that, there is nothing else that you can do, for everything is in the hands of the Almighty.
Just like before that.