It's that whole rhyme thing. Plus, the convenient contrasting personalities/roles in the clique.
Notice that sometimes the second ship appears as one of the most matured of the class(eg:Kisaragi,Shirayuki,Hibiki,Shigure)and some of them are not so(eg:Nenohi,Makigumo)not sure about Asakaze,Shikinami,Ooshio,Shiranui and Teruzuki though.Is this a trend I am noticing?
Around when I started out, for some reason I believed that Arashio was the second daughter.Actual second daughter