Using evidence from the playThe Importance of Being Earnestas far as the traits of all four characters involved, Jack with Gwendolen, and Algernon with Cecily, the chances of any of the two couples to succeed is quite low if we judge the relationships from the moral and sanctimonious standards of middle class Victorian London.
Yet, judging from the perceptions of each character regarding love, commitment, and morality, Jack is the least likely character to fail because all throughout the play he had demonstrated a consistent devotion towards Gwendolen. After all, the play itself is about Jack's struggle to comply with Lady Bracknell's request of "finding a relation" in order to accept him as a future son-in-law. Therefore, Jack is the type of man that is willing to put up with humiliation, and seems to be willing to make a sacrifice, albeit small, to secure his chance to marry Gwendolen.
It is true that Gwendolen is like her cousin Algernon in terms of how fickle she is about commitment. After all, she is the character who said that if her mother prevents her from marrying Jack she wouldn't mind marrying many other men instead.
..although [Lady Bracknell] may prevent us from becoming man and wife, and I may marry some one else, and marry often..
Yet, Gwendolen is also seemingly devoted to Jack, even after she learns that his name is not really Ernest, which is the primary reason why she loved him in the first place; quite a superficial reason but the play is a trivial comedy nevertheless.
In contrast, Algernon is an incurable bachelor who falls in love with love, and with all the pleasures of life. Despite his claim of loving Cecily, the fact remains that their instant connection has all the wrong foundations: the way in which they are introduced is fake because he pretended to be someone that he is not, and Cecily had made up a relationship with this persona that had come to exist only in her imagination. Therefore, Algernon and Cecily are literally two very imaginative people with a tendency of using their imagination to re-create their reality; this is hardly the stuff a highly-committed relationship is made of.
Hence, Jack is willing to do much more for love and for the woman that he loves than Algernon ever would imagine doing. Similarly, Gwendolen's age and worldly knowledge can work on her behalf to make her a very good and sophisticated wife. The pair of Jack and Gwendolen is definitely the better-matched pair.
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