When Mercutio is looking for Romeo, he uses Rosaline’s name to draw him. He does not know that Romeo is over Rosaline, and is now in love with Juliet.
When Mercutio is looking for Romeo, he makes a bawdy speech about Romeo being a lover that seems to indicate he still thinks Romeo is in love with Rosaline.
I conjure thee by Rosaline's bright eyes.
By her high forehead and her scarlet lip,(20)
By her fine foot, straight leg, and quivering thigh (Act 2, Scene 1, p. 37)
Mercutio uses this locker room talk to bring Romeo out. He says he can “conjure” him up by mentioning Rosaline’s beauty. It makes sense that Mercutio would think that Romeo is in love with Rosaline still. After all, he was the last time Mercutio saw him. This is a reminder to the audience of how quickly Romeo’s affections have changed.
At this point, there are several good reasons for Romeo to avoid his friends. First of all, Mercutio is obviously interested in ribbing him about Rosaline dumping him. Second of all, Romeo’s interest in Juliet is forbidden. He is not supposed to talk to her, let alone fall in love with her, because their families are feuding. Romeo is also not sure at this point if Juliet actually loves him back.