Doctor Faustus makes his motivation quite clear in his introductory soliloquy, and the insight he gives is confirmed by what the Chorus had said already. The Chorus orients the audience/reader to the play by stating that Faustus earned his academic Doctoral degree in "theology" (religion) only to become overinflated and conceited in his opinion of himself. This leads him to a moral and spiritual downfall in which he aspires to becoming and knowing what is beyond him.
In a Biblical parallel and allusion, Faustus is connected to Lucifer's fall from Heaven: Lucifer aspired to be like God; he aspired to be and know that which was beyond him. The Chorus describes Faustus's similar fall with a classical Greek allusion to Icarus, who, while escaping the Minos, flew on wings made of wax by his father past the Sun. Against his father's warnings, Icarus felt unlimited in his power and, in his arrogance, flew too close to the Sun thus melting his waxen wings and plunging himself to his death. This, by the way, foreshadows the tragic conclusion to Faustus's story: he is not saved at the last minute.
The fruitful plot of scholarism grac'd,
That shortly he was grac'd with doctor's name,
Excelling all whose sweet delight disputes
In heavenly matters of theology;
Till swoln with cunning, of a self-conceit,
His waxen wings did mount above his reach,
And, melting, heavens conspir'd his overthrow;
For, falling to a devilish exercise,
And glutted now with learning's golden gifts,
He surfeits upon cursed necromancy;
Nothing so sweet as magic is to him,
To recap, according to the Chorus, Faustus's motive is arrogance and vanity and "self-conceit" that leads him to aspire to things beyond and "above his reach." In his opening speech, Faustus confirms and elaborates upon this same assertion of motive.
Faust declares that he shall devote the rest of his life not to theology but to necromancy: magic. He declares that he shall command the wind and clouds. He declares that his "dominion" shall stretch as far as the mind can reach, that he shall be all-powerful, or "omnipotent." He is motivated by his desire to be a "mighty god" and "gain a deity." These are objectives that are surely beyond his reach and surely the product of "self-conceit" and arrogance.
These metaphysics of magicians,
And necromantic books are heavenly;
Lines, circles, scenes, letters, and characters;
Ay, these are those that Faustus most desires.
O, what a world of profit and delight,
Of power, of honour, of omnipotence,
Is promis'd ...
All things that move between the quiet poles
Shall be at my command: ...
... 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.