Capulet turns on Juliet because she states that she will not marry Paris and, in fact, hates Paris. Capulet believes he has found Juliet a very worthy husband that abides by the belief that children should be grateful and obey their parents. He believes he is doing her a favor by marrying her off so well. However, since Juliet is already married, she cannot be anything but terrified by the news. She cannot exactly come out and tell her parents right then that she is already married.
Not proud you have, but thankful that you have.
Proud can I never be of what I hate,
But thankful even for hate that is meant love. (Act 3 Scene 5 Lines 146-148)
She acknowledges that her father meant well and states that she is very grateful for his trying to make her happy and marry her into a good position. Yet she cannot marry again so she must find a way to talk her father out of the marriage idea.
Capulet takes it defiance and is highly angered.
But fettle your fine joints 'gainst Thursday next
To go with Paris to Saint Peter’s Church,
Or I will drag thee on a hurdle thither.
Out, you green sickness, carrion! Out, you baggage! (3.5.153-156).
Juliet then seeks to earn his good favor again through her words. She tries to explain that she means no disrespect, but her father’s temper does not allow him to hear reason. All he can do is demand obedience and yell insults at her for daring to defy his wishes and not be grateful of the good he is trying to do for her.