For non Lost fans: Desmond believes he can sometimes see the future; he can see that Charlie will die.  He also believes he is responsible to push a button; if he does not, the world ends.

He meets an Oracle, who also sees the future-- e.g. she knows the man in the red shoes (Wicked Witch?) will die-- but explains that it is futile to try and save him, because "the universe has a way of course correcting," that is, destiny will find another way to take his life.  Save him from the scaffolds, tomorrow he gets hit by a bus.

So Desmond now knows Charlie is going to die.  He's prevented the death twice, but he's feeling the futility of it all-- destiny is coming, hungry, and it will be satisfied.

Desmond has to choose whether or not to continue trying to delay the inevitable.  Desmond is the Cowardly Lion.  He's afraid, he's confused.

So be it.  But the important question is not whether Desmond will continue to try to delay Charlie's death, or just give up.  The real question is why Desmond actually believes such a choice exists.  How does he think he knows the future?   Anyone else in his shoes would have come to a very different, more logical, conclusion: this is insane.  What, he can predict the future?  Worse: what, he's the only reason Charlie is alive?  He's so-- necessary?  Isn't that narcissism?

You might say, "well, in the logic of the show, Desmond knows he can predict the future, and so he tries to save Charlie."  Wrong, and this is exactly the point.  Remember how he correctly predicted the outcome of the soccer match-- but was wrong about which soccer match he predicted?  Sure, ok, Charlie's going to die.  When?  2014?  

Was Charlie really going to die in the water? Was he really going to get hit by lightning? Is Desmond actually saving him, or is it all-- wishful thinking?

So what makes Desmond's story so powerful is not simply that he chooses to save Charlie again and again; what's more important is that he chooses to believe in a life that where he must make such a choice. 

If Desmond knew he could predict the future-- if it was a fact that he could predict the future-- then saving Charlie would have little moral heroism.  Any fool a step up from absolute evil would have tried to prevent a horrible outcome if he knew for certain what was going to happen.

What made Desmond worthy of admiration was, exactly, that he did not know for sure he could predict the future. He took it on faith that he could, and then proceeded to live his entire life based on this single, faith based, assumption.  He put his money where his mouth was.

Desmond took a leap towards faith, not a leap of faith.  He didn't have faith to leap with.  He went towards it, picked it. He didn't know the button needed pushing, and so, like a soldier, took responsibility to push it.  He took on faith that the button needed pushing and then furthermore decided it was his responsibility to push it, defying logic and sanity and evidence and, well, everyone else.  The action wasn't just heroic; it was heroic and defining.

He decided that he was going to give his life meaning, importance, even if it was the most insane, solitary, depressing meaning available; and at the great risk that he could be wrong, a life wasted.

If you know for certain God exists, there is nothing noble in believing in him.  It's only when you take the great gamble to live your life, your entire life, as if He exists-- at the risk of being wrong, at the expense of an easier, happier life-- that you define yourself as something greater.   And once Desmond so defined himself, arbitrarily, he was able to take the next steps necessary to grow, evolve.

And he does grow.  Almost immediately, he becomes a better man: after accepting the logic in a worldview in which he can predict destiny's path,  he then also accepts a responsibility-- saving Charlie-- that he also knows is futile.  He knows he has to fail, eventually, but he's going to keep doing it anyway, because he thinks he can transcend his own logic.  

And because of this, he will succeed.

That's the Key in the Failsafe.   How can you say that the button absolutely must be pressed, if there is a simple way of bypassing it?  If pushing the button is the only way to save the world, why is there even a failsafe in case you stop pressing it?  It's a tricky answer: it's because while the button was being pressed, the Failsafe did not exist.  The Failsafe represents Desmond not failing, even in the face of reality.  It isn't a pre-existing backup.  Desmond's existence created the failsafe.  Metaphorically, the Failsafe didn't exist until Desmond took the final step: after accepting the futility of his actions, but deciding to do them anyway, he then decided he would not fail.  

No matter what happens-- no matter how certain the reality-- he knows he can not fail, and so he will always will a solution.

It's the religious existentialist position.  Kierkegaard didn't think logic and reason was going to get us to any absolute truth, and it certainly wasn't going to help us understand an Unintelligible God (which is the same thing.)  You just have to go to faith, embrace it, create a life using it as a postulate, and move forward.

It's narcissism done the right way.  And, I suspect, it's the secret to a meaningful life: picking an existence that is of value to more than just yourself, even if that existence defies the logic of reality-- your biology, your environment, and, of course, everyone else.  And once you have chosen who you want to be, once you have defined the parameters of this life, you force it to be true, as real as any gene or social factor. And know that once you have invested your life in this identity, this existence-- all or nothing, even in the face of the doubt and terror that accompanies your "rational" self--- it will be impossible to fail.

...as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.