What does Romeo mean by "He jests at scars that never felt a wound"?
In Act II, Scene 1, Mercutio is joking with Benvolio about Romeo's love-sickness for Rosaline, saying,
I conjure thee by Rosaline's bright eyes.
By her high forehead and her scarlet lip,
By her fine foot, straight leg, and quivering thigh,
And the demesnes that there adjacent lie,
That in thy likeness thou appear to us!
Mercutio does not realize Romeo is already over Rosaline, as he fell in love at first sight with Juliet. Romeo's friends have no chance of finding him, although he is just on the other side of the wall. It would never occur to them that Romeo would have the temerity to climb that wall and leap down within the Capulet orchard, because (1) the Capulets are Romeo's family's worst enemies, and (2) they do not know he is in love with Juliet.
The words, "He jests at scars that never felt a wound" occur at the very beginning of Scene 2, but Romeo is referring to all the "jests" made about him in Scene 1. Romeo is implying Mercutio is able to laugh at a man who is in love because he has never been in love himself. By "scars," Romeo is further implying that he has been in love more than once and has recovered, just as he got over his love for Rosaline.
Although Romeo seems to fall in love and to recover pretty easily, we still feel that he is a stronger character than his friend Mercutio because Romeo is not afraid to fall in love, just as he is not afraid to climb the Capulets' wall to look at Juliet. Romeo knows from past experience how painful it can be to fall in love, yet nevertheless falls in love again when he sees Juliet at the Capulet party. Mercutio can only think love is funny because he has never experienced the pain of love. Everyone who has ever been in love knows it can be a very painful experience. Shakespeare's play will prove how painful love turns out to be for the young lovers Romeo and Juliet.