Why did shakespeare choose to begin Act Two with Mercutio and Benvolio rather than Romeo?

Mercutio and Benvolio are chosen to open Act Two because Shakespeare wished to keep up the lightheartedness of the play's opening. These characters are more upbeat than Romeo, who is in deep love with Juliet and about to do something extremely risky because of it. Thus, their light-hearted banter contrasts with the dramatic conflict that is rising to heighten suspense.