Lady Capulet misunderstands the reason for Juliet's sorrow.  Lady Capulet thinks that Juliet is upset about Tybalt's death, but she is more concerned about Romeo who has been exiled to Mantua or else he will be killed.  Juliet refuses to marry Paris because she is already married to Romeo, which she can't tell her mother.

Lady Capulet has no idea what has been happening in her daughter's life (or heart).

The last conversation Lady Capulet had with Juliet prior to the party was about marriage (to Paris). At that time, Juliet had told her it was an honour she dreamed not of, meaning it was not something that she aspired to or dreamt about. Lady Capulet rightly thinks that Juliet is devastated over the cruel murder of her cousin Tybalt, which, in part, she is. What Lady Capulet doesn't know is that Juliet, who said she wasn't interested in marriage, has already met someone, fallen in love, and married (without telling either of her parents). In Lady Capulet's defense, the misunderstanding is not at all helped by Juliet, who plays word games with her mother, telling her truths while leading her to continue with her mistaken interpretation of the reasons for Juliet's sadness. Juliet says she would rather marry Romeo (her cousin's killer) than Paris, which is true but not at the level her mother takes it. She also says she wants to bear the poison to Romeo herself, again leading her mother to think that her hatred for Romeo is intense; we know she just wants to see him and, to intervene against her family's attempts to murder him in revenge.
Finally, Lady Capulet herself seems quite enamoured with Juliet's suitor, Paris (she says Verona's summer has not seen so fair a flower as him); it is also possible, therefore, that she simply cannot conceive of Juliet falling for anyone else. In her mind, there could be no other logical reason for Juliet's state of mind than the immediate one before them.

Lady Capulet thinks Juliet is crying because of Tybalt's death. He was her cousin. Juliet is actually crying because it was Romeo who killed Tybalt, and as a result, he has been banished from Verona. As a result of thinking Juliet is crying over Tybalt, her mother tells her she's going to send someone to kill Romeo. Then her father thinks having Juliet marry Paris right away will cheer her up.

In act 3, scene 5 of Romeo and Juliet, Lady Capulet enters Juliet's chamber to find Juliet completely distraught. She wisely remarks upon excessive grieving for a lost loved one:

Some grief shows much of love,/But much of grief shows still some want of wit (3.5.74-75)

When Juliet asks that she be allowed to weep for such a deeply felt loss, Lady Capulet, in a moment of dramatic irony, retorts that Juliet

weep'st not so much for his [Tybalt's] death/As that the villain lives which slaughtered him.

While Lady Capulet believes that Juliet cries because the murderer still lives, she does mourn Romeo, but in a different way:

...I do with all my heart,/And yet no man like he doth grieve my heart (3.5.82-83)

Juliet...