When romeo and juliet first meet in act 1, scene v, a sonnet structure is used because?

In Act II, Scene III, of Romeo and Juliet, Romeo visits Friar Lawrence and declares his love for Juliet. He then asks the Friar if he will marry them:

I’ll tell thee as we pass, but this I pray:

That thou consent to marry us today.

The Friar is shocked that Romeo wants to marry Juliet because he claimed to be madly in love with Rosaline, a silent character in the play. In fact, Romeo was quite love-sick and it was the Friar who tried to convince Romeo to let Rosaline go because she did not reciprocate his feelings. The Friar then goes on to scold Romeo because he did not want him to abandon his love for Rosaline only to go on to fall in love with another woman. However, when he realizes that Romeo is serious about Juliet and that she reciprocates his feelings, he agrees to marry them. He also recognizes that this marriage is an opportunity for the two warring families to be reconciled:

For this alliance may so happy prove

To turn your households' rancor to pure love

the first step is to convey the appropriate emotions.