Goethe's Faust is rife with troubling opposites, not the least of which is its eponymous character's tortured relationship with beauty. As he strikes his bargain with Mephistopheles, Faust fervently disavows the pleasures of "mere quiet satisfaction", proclaiming that "if ever [he] lie down in sloth and base inaction,/Then let that moment be [his] end": that is, should he settle on a given moment as one that he wishes to endure forever, his life and soul are forfeit (Faust, 1691-3). He desires all the pleasure and beauty that the world has on offer, but in mutable form: nothing that will last forever, but instead fleeting, transitory things like "a meteoric fame/That fades as quickly as it came" or "fruit that rots before it's plucked/And trees that change their foliage every day" (1684-7). Even his quest for the ideal woman is not immune to this caveat; and it is mostly for this reason that Helen of Troy's "ancient proverb", "that beauty weds not long with happiness" is fulfilled time and time again (9939-40).
Faust's quest for "the very paragon of femininity" begins to take shape in the witch's kitchen, where he sees the image of a beautiful woman in a mirror and is captivated by her (2602). In an aside, Mephistopheles off-handedly adds that the witch's potion that Faust drank will so affect him that "any woman will be Helen to him" (2604). The Helen to whom he refers, of course, is Helen of Troy -- the legendary "face that launched a thousand ships", and the classical ideal of feminine beauty. Faust does encounter Helen herself later on; but the first woman he sees (and thus the woman with whom he subsequently falls deeply in love) is Gretchen.
In contrast to Helen's almost mythical classic beauty, Gretchen is a young German girl who is not at all idealised by legend the way that Helen is. Goethe may have intended for her to have represented a romantic ideal, as Faust rejects his classical education to instead pursue pure romantic feeling wholeheartedly. She is somehow more real and tangible than the seemingly-unattainable ideal of Helen; his love for her is certainly reciprocated in full, and it is made all the more profound by virtue of the good-natured simplicity of her character. Yet even though her love for him is almost saccharine in its sincerity -- even though she is hardly able not to wholly surrender herself to her lover, even when his will puts her own family in danger -- Faust's restlessness makes him dissatisfied with their situation, asking for more and more until Gretchen is eventually driven to ruin, with her brother dead by Faust's hand, and her mother and infant by her own.
Gretchen herself is partly at fault for her downfall. By proving that she truly would do anything at all for Faust's sake, to the point of poisoning her own mother with a sleeping draught against her better judgement so that they might have time alone together undisturbed, she becomes an active participant in her own manipulation, and therein lies her tragedy. This is hardly a legitimate excuse for Faust's abandonment of Gretchen -- but the beauty of their love for each other could not remain tied to the happiness it brought them indefinitely because by the terms of Faust's bet with Mephistopheles that would constitute a defeat, and would lead to Faust's condemnation and eternal damnation.
All beauty is not lost for Faust and Gretchen, however: perhaps there is some redemption in the fact that her soul is saved and spirited off to Heaven for an eternity of happiness upon her death, despite Mephistopheles' certainty that "She is condemned!" (4611). Whether Helen's proverb is meant to address outward physical beauty -- the sort that Faust saw in the witch's mirror, in Gretchen, and in Helen -- or inward beauty, like that of their love, is unclear; though perhaps it doesn't matter. Though she was not "the loveliest of women" as Helen is (9482), she was beautiful; and in the end her beauty only brings her misfortune. In her case, too, the "ancient proverb proves itself" (9939-40).
Despite the loss of his first love, happiness with an ideal woman is not out of Faust's reach or ambition. He continues to pursue it with single-minded determination, settling on Helen of Troy, the classical ideal of feminine perfection, as the woman he must have. She is mythical and almost unreal, appearing only as a shade until Faust takes Mephistopheles and goes to Greece himself to win her favour, whereupon he takes her away to Arcadia and the two of them have a son, Euphorion.
If Faust's romance with Gretchen was a thoroughly modern pursuit, then his relationship with Helen is an attempt to return to his classical roots and capture the idealised eternal in the temporal, in his own experience. His love for Helen is just as powerful as his love for Gretchen, if it is not made even more so by her limitless beauty, the very ideal that he had seen in the witch's mirror. Their son is the culmination of that love, as Helen says: "Love uniting man and woman/Shapes a joy of two made one;/Two with rapture more than human,/Are made three; this love has done" (9699-9702). Faust himself adds that he and Helen are now bound in a "sacred union", and wonders whether this idyllic existence of beauty and happiness together is not "fate's design" (9705-6).
This seems perilously close to the "beautiful moment" forbidden by the wager, and indeed it strays too closely -- Euphorion, lost in some sort of physical ecstasy, fails to heed Faust's warning that "They fall to ruin/Who leap so wildly" and perishes, falling to the ground at his parents' feet and then vanishing before their eyes. There is no question here of what is meant by Helen's proverb: as Euphorion dies, Helen cries out that "the bond of love is severed now, and so of life;/Bewailing both, [she bids] a sorrowful farewell" to Faust, before vanishing herself, suddenly bereft of happiness and no longer able to continue living after the untimely death of her son.
Beauty and happiness are not necessarily opposed in either case, but for Faust himself they may as well have been. Finally reaching the perfect beauty and perfect love he sought would represent a culmination of everything he wanted, resulting in a moment too beautiful to be let go. Naturally, this would spell disaster for Faust: his wager lost, his "time [would] come to an end", and he would be damned.
For earlier writers like Christopher Marlowe, trying to drive home a point about morality and the dangers of ambition, Faust's damnation might have been the intuitive ending. But as Goethe's Faust proceeds, that sort of conclusion becomes not merely contrived and unsatisfactory but entirely impossible. In fact, it was as good as established from the outset that Faust would not lose his wager: the Prologue in Heaven has God Himself expressing perfect confidence in the fact that Faust would prevail -- that "a good man, in his dark, bewildered stress,/Well knows the path from which he should not stray" -- and that Mephistopheles would be forced to admit defeat (327-29).
If Faust's God is the omniscient Christian God, He could not possibly be mistaken about such a matter -- thus Faust was destined to triumph. The segregation of happiness and beauty was necessary, in order for God to "lead [Faust] into clarity" out of confusion (308) -- and even though the two were never opposed, they can only be truly united after Faust's death, when he has been found worthy and is reunited with Gretchen in Heaven.
Goethe. Faust: Part One & Part Two, trans. David Luke. Oxford: Oxford University Press, 1998. | 
