Well think about it, Benvolio and Mercutio are Romeo's best friends. They are worried about him, because firstly he is in love with Rosaline - this love in unrequited. This makes Romeo very depressed for a long period of time. So, of course they are worried about him. Also, after Capulet's party, Romeo mysteriously disappears, which leaves both Mercutio and Benvolio worried for him. As well as the fact that Tybalt discovered that Romeo had gatecrashed the party and now is looking for vengeance. Hope this helped.
