In Act 3, Scene 2, Juliet is faced with the terrible realization that the man she has just married has also just murdered her cousin. Not only that, she is suddenly faced with the terrible idea that a man she thought was wonderful may not actually be so very trustworthy. Therefore, Juliet is not just telling herself not to be upset, but rather re-convincing herself that she really can trust Romeo.
We see Juliet's sudden doubt in Romeo expressed through her long string of oxymora. When she first met Romeo, she used his handsome looks to judge that his soul was just as beautiful, but now she believes she may have been deceived, as we see in her lines:
O serpent heart, hid with a flow'ring face!
O, that deceit should dwell
In such a gorgeous palace! (III.ii.76-88)
However, when her nurse begins to speak poorly of Romeo as well, Juliet has a moment of awakening. She asserts that Romeo "was not born to shame" (96). She also decides that she should not speak ill of her husband, simply because he is her husband. She next rationalizes reasons for why Romeo might have killed Tybalt in order to shed better light on what happened. She reasons that if Romeo had not killed Tybalt, Tybalt would have killed Romeo. Understanding that Romeo must have killed Tybalt out of self-defense helps her to see things differently and helps her restore her trust in Romeo. Hence, she tells her herself that she should not be upset over the fact that Romeo has slain Tybalt because Romeo would have been killed if he had not. Plus, understanding that helps her to love Romeo and trust him once more.