The Mystery of 'Die before you Die' is this:
That the gifts come after your dying and not before.
Except for dying, you artful schemer,
No other skill impresses God. One Divine gift
Is better than a hundred kinds of exertion.
Your efforts are assailed from a hundred sides,
And the favor depends on your dying.
The trustworthy have already put this to the test.

                                                           —Rumi

Sufism In A Nutshell

Allah has said:

                        And I did not create the jinn and mankind except to worship Me.  —Qur'an 51:56

Man was created solely to worship Allah. And the purpose of worship is twofold, dhikr Allah, and the curbing of the ego.

In Sufism, dhikr Allah becomes continuous from morning to night, and the curbing of the ego is replaced with the death of the ego, in respect of the commandment of the Messenger of Allah, who said:

                                           Die before you die.  —Hadith

And said also:

Whosoever desires to see a dead man walking upon the face

                             of the earth, let him look at Abu Bakr.  —Hadith

And death is desirable because in death there is unveiling (kashf). Allah has said in respect of death:

Surely you were heedless of this, so We removed your cover from you, and

                  today your eyesight is sharp. —Qur'an 50:22

Sufism is the science of certainty (yaqin). Unveiling (kashf) is that which grants certainty.

And thus did We showed Abraham the kingdom of the heavens and the

                     earth and that he might be of those who have certainty. —Qur'an 6:75

For this reason, a sheikh is required for the sheikh is the slayer of egos in that he slays the egos of his disciples. It is through the sheikh that the commandment of Allah's messenger is fulfilled.

This is Sufism in a nutshell, so to speak. The rest is commentary.