In which ways do both Romeo and Juliet change (in contrast to before they meet). Do they make things better or worse?
The change that Romeo and Juliet effect is bittersweet-both better and worse. For, their deaths are tragic, yet these losses bring about the end of the Capulet/Montague feud.
As far as their individual characters, both Romeo's and Juliet's personalities seem even more impulsive than they are at the beginning of the play. While he is reluctant to give "liberty unto thine eyes./Examine other beauties" (I,i,193-194) as Benvolio suggests in Act One and continues to bemoan his loss of Rosaline, in the final act Romeo does not bother to verify the information that he receives about Juliet's death by contacting Friar Lawrence who would know the truth. Instead, he rushes to the apothecary; he then hastens to Verona and Juliet's family's crypt where he again rushes headlong to his death before even a few minutes elapse.
Juliet, too, becomes more impetuous. At her party in Act I, she cautions Romeo "you do wrong your hand too much,/Which mannerly devotion shows in this..." and in Act II suggests that he hurry away after their wedding night lest the daylight capture his view by others, she later rages in ambivalence in Act III: first against Romeo, then against Tybalt after she learns of her cousin's death. Then, when told by her parents that she must marry Paris, she rashly tells falsehoods and hopes the Friar can save her or she will "myself have power to die" (III,v,242).
When she does arrive at Friar Lawrence's cell, she pulls a dagger out before the priest, telling the friar that she will kill herself if he does not help her somehow. Such behavior belies a mature young woman who is capable of the demands of marriage. With only slight anxiety, she quaffs the vial given her by Friar Lawrence, an amateur herbalist, at best. When she awakens, she hastily assumes the worst and in quick despair with no regard for her parents, kills herself--"O happy dagger" (V,iii,(169)--because her Romeo is dead.
Clearly, the theme of the impetuosity of youth is depicted in the characters of young Romeo and his Juliet.