Mercutio and Benvolio were looking for Romeo. Why do they think he is hiding from them?

Benvolio and Mercutio are unaware of Romeo's feelings for Juliet at this point.  They believe he is still in love with Rosaline, who does not love him in return and intends to join a convent.  When Benvolio and Mercutio are unable to find Romeo following their crashing of the Capulet party, they assume that he is keeping himself away from them because his heart is still broken and he does not want company.  They believe that he simply wants to be left alone because he is sad from dwelling on the love he cannot have.  They give up on their quest to find him, since they realize that he does not mean to be found.