Why does Nerrisa marry Gratiano?

The relationship between Nerissa and Gratiano is a good deal more traditional than that between Bassanio and Portia. In keeping with the prevailing standards of the time, Gratiano is the dominant partner in the relationship, whereas Nerissa is generally demure and submissive. At the same time, there is undoubtedly great love between them, though it's noticeable that their decision to marry is dependent on Bassanio choosing the correct casket and so being able to marry Portia. Once he does so, however, Gratiano wastes no time in getting hitched to Nerissa. This shows his impulsive side. Gratiano is presented throughout the play as a bit of a fool, a man seemingly incapable of taking anything seriously.
Nerissa goes along with this. Though she's quite submissive, as mentioned earlier, she does display some degree of independence; it is she who insists that marriage to Gratiano is conditional on Bassanio and Portia doing likewise. Gratiano and Nerissa's forthcoming nuptials thus parallel those of their social superiors. From a dramatic standpoint, the double marriage makes sense because it allows Shakespeare to say something important about love and marriage. But from Nerissa's perspective it's difficult to see why she'd marry Gratiano under such unusual circumstances, even if she does love him. Our perplexity is further reinforced by Gratiano's giving away Nerissa's ring just as Bassanio does with Portia's. But Nerissa follows her mistress in everything, and getting married is apparently no different.