Why is Romeo alone at the beginning of Act 2 of Romeo and Juliet?

Having just left the Capulet party, Romeo is nonetheless alone because he has decided to leave his friends and to seek out Juliet again.  In a dangerous move, Romeo opts to climb the walls of Juliet's courtyard.  This move will put him outside of her room just as Juliet is talking to herself out on her balcony.  This will set up the famous balcony scene that occurs in Act 2, Scene 2.

Romeo's friends, who...

Having just left the Capulet party, Romeo is nonetheless alone because he has decided to leave his friends and to seek out Juliet again.  In a dangerous move, Romeo opts to climb the walls of Juliet's courtyard.  This move will put him outside of her room just as Juliet is talking to herself out on her balcony.  This will set up the famous balcony scene that occurs in Act 2, Scene 2.

Romeo's friends, who are unaware of Juliet, are likewise unaware of why Romeo has left.  Indeed, Mercutio initially states that Romeo has gone home to bed, until Benvolio corrects him by stating that Romeo jumped over the wall.  Mercutio (wrongly) deduces that Romeo must have returned to try to unite with Rosaline and proceeds to "conjure" Romeo by shouting to Romeo about Rosaline's legs, thighs and areas near to her thighs (this is Mercutio's sexual innuendo at its best).