In Romeo and Juliet, does Benvolio's wish for Romeo come true? Why is this a serious problem?
I'm not quite sure what you mean by Benvolio's "wish for Romeo". Perhaps you're referring to the part, in the two of them's first scene together, when Romeo is whinging on about being in love with Rosaline.
Benvolio tells him he should forget about her, and find someone else to be in love with. And then Peter comes ambling in with the guest list for the Capulet party that evening, and Benvolio sees his chance to get Romeo's mind onto other women:
At this same ancient feast of Capulet's
Sups the fair Rosaline whom thou so lovest,
With all the admired beauties of Verona:
Go thither; and, with unattainted eye,
Compare her face with some that I shall show,
And I will make thee think thy swan a crow.
When Romeo sees some other woman at the party, he'll forget about Rosaline, Benvolio thinks. And of course, this is exactly what happens - only the girl he happens to choose is another, supposedly unavailable Capulet: Juliet.
The Montagues and Capulets don't get on, and have an "ancient grudge" against each other: and so it's very unlikely that a relationship between Romeo (Montague) and Juliet Capulet is going to be a success. That's why it's a serious problem when Romeo falls for another Capulet!