This is a complex portion of Goethe's poem. Understanding Faust's motive is dependent upon (1) understanding his quest and unrest as revealed in "Night" and "Before the City-Gate"; (2) understanding the wager between The Lord and Mephistopheles in "Prologue in Heaven”; (3) and understanding the importance of the theme of change Goethe's Faust. This theme is introduced in the Prologue when The Lord says to Mephistopheles,
Sees not the gardener, even while buds his tree,
Both flower and fruit the future years adorning?
This establishes the idea that within one state of being, another state of being--the next state of being--lies waiting. In other words, change lies waiting as the natural progression of being. The wager The Lord strikes with Mephistopheles is based on this idea. The Lord replies to Mephistopheles' suggestion of a bet ("What will you bet?") by saying that a good person will always be led by "an instinct of the one true way." In relation to change as the natural progression of being, The Lord's reply means that within Faust lies the blossoms and the fruit that follow the buds: "Both flower and fruit the future years adorning."
Faust's yearning, as he expresses it to Wagner in "Before the City-Gate," is to have a unity between what one knows and what one uses and to have a unity between the two parts of his soul:
That which one does not know, one needs to use;
And what one knows, one uses never. /... /
Two souls, alas! reside within my breast,
And each withdraws from, and repels, its brother.
Faust feels great unrest within himself; it is this unrest that compels him in "Night." It is this unrest that leads him--that motivates him--to set the terms of a wager with Mephistopheles. His quest is to find that thing that known is needed and that needed is known--that one thing that unifies. This is why he turns to magic and the cosmos--he expects to find in it the power that unifies and unites:
O happy he, who still renews
The hope, from Error's deeps to rise forever!
The terms of the wager Faust makes with Mephistopheles reflect this yearning and quest. He says, in essence, if ever Mephistopheles makes his divided nature quiet; if flattery can make him view himself with pleasure (replacing yearning); if enjoyments can turn his mind from his quest; when Faust calls for time to stop, then Mephistopheles can claim Faust. In other words, the wager may be paraphrased as, "Soothe my yearning and satisfy my quest and make me desire to live, then you win my soul." Note how this reflects the wager The Lord made with Mephistopheles: the flower and fruit are bound together, and a good person “Has still an instinct of the one true way.” Faust's wager is:
When on an idler's bed I stretch myself in quiet, ...
Until, self-pleased, myself I see,—
Canst thou with rich enjoyment fool me,
Let that day be the last for me!
The bet I offer. / ... /
When thus I hail the Moment flying:
"Ah, still delay—thou art so fair!"
Then bind me in thy bonds undying, ....