Dr. Faustus is on the verge of damnation throughout the play. Explain what desires leads him to make a pact with the devil, what kind of thinking keeps him with the devil throughout the action of the play and what final actions bring about his destruction.

Christopher Marlowe's The Tragical History of Doctor Faustus was written between 1589 and 1592. Two versions of the play exist—one from 1604 and another from 1616—but the essential elements of the rise and fall of Doctor Faustus exist in both versions.
Doctor Faustus was written at a time of increasing belief in the freedom and independence of the individual and a questioning of the role of religion, especially scripture and religious dogma, in people's lives.
Another playwright put it succinctly in a play written just a few years later:

There is nothing either good or bad, but thinking makes it so. (Hamlet, 2.2.257–258)

Faustus could choose to use his mind to benefit humanity, as a philosopher, lawyer, theologian, scientist, or physician, but he is driven by his pride and ambition to rule the world and enjoy its excesses. Faustus chooses necromancy—sorcery and the dark arts—to further his goals.

Ay, these are those that Faustus most desires.O, what a world of profit and delight,Of power, of honour, of omnipotence,Is promised to the studious artisan!All things that move between the quiet polesShall be at my command. Emperors and kingsAre but obeyed in their several provinces,Nor can they raise the wind, or rend the clouds;But his dominion that exceeds in this,Stretcheth as far as doth the mind of man;A sound magician is a mighty god:Here, Faustus, tire thy brains to gain a deity. (1.52–63)

Faustus makes an incantation to summon up the dark spirits, and he conjures up Mephistophilis, who has reservations about what Faustus wants to do.

MEPHISTOPHILIS: O, Faustus! leave these frivolous demands,Which strike a terror to my fainting soul. (3.85–86)

Faustus persists, and in due time, he makes a deal with Mephistophilis: his soul in exchange for twenty-four hours of unlimited power, wealth, and pleasure.
During his twenty-four hours, Faustus has cause to doubt his deal with the devil. It appears that Faustus didn't read the fine print in the agreement that he signed in blood about having a wife, for example. A good angel appears from time to time to remind Faustus that it's never too late to repent, and at one point, Faustus agrees to repent, but he's reminded of his agreement:

GOOD ANGEL. Never too late, if Faustus can repent . . .
FAUSTUS. Ah, Christ, my Saviour,Seek to save distressed Faustus' soul!
[Enter Lucifer, Belzebub, and Mephistophilis.]
LUCIFER. Christ cannot save thy soul, for he is just;There's none but I have interest in the same. (5.85, 90–94)

Mephistophilis, Belzebub, and Lucifer keep Faustus distracted until it's time for Mephistophilis to take Faustus's soul to hell.
Faustus regrets his bargain with the devil. An old man appears and gives him one last chance to repent.

OLD MAN. I see an angel hovers o'er thy head,And, with a vial full of precious grace,Offers to pour the same into thy soul:Then call for mercy, and avoid despair. (12.50–53)

But Mephistophilis threatens Faustus:

MEPHISTOPHILIS. Thou traitor, Faustus, I arrest thy soulFor disobedience to my sovereign lord;Revolt, or I'll in piecemeal tear thy flesh. (12.63–65)

In despair, Faustus relents and bargains for one more thing.

FAUSTUS. One thing, good servant, let me crave of thee,To glut the longing of my heart's desire,—That I might have unto my paramourThat heavenly Helen which I saw of late,Whose sweet embracings may extinguish cleanThese thoughts that do dissuade me from my vow,And keep mine oath I made to Lucifer. (12.79–85)

And in a "twinkling of an eye" (12.87), there she is: Helen of Troy.

FAUSTUS. Was this the face that launched a thousand ships,And burnt the topless towers of Ilium? (12.89–90)

They kiss. Helen's lips "suck forth my soul; see where it flies!" (12.92), and Faustus is condemned to eternal damnation.