There are a couple of different explanations as to why we see Juliet tell her mother one thing about her desires towards marriage yet see her act with completely different inclinations privately.
The first explanation is that Juliet may not have been being completely honest with her mother. Even though Juliet is young, only twelve, she has been out in society for a while, probably since her twelfth birthday. If she had not been out in society, Paris would not have asked for her hand in marriage, not once but twice. Since Juliet is out in society, she knows that she will be receiving offers of marriage. She knows that her mother will soon be talking to her about her marriage prospects and she is not looking forward to the idea of an arranged marriage. Therefore, when Juliet tells her mother that marriage "is an honour I dream not of" (I.iii.70), while it may be true that she is not looking forward to the idea of an arranged marriage, like a typical romantic, Juliet has thought more about romantic love and marriage than she is willing to tell her mother. Hence, we may be able to assume that Juliet is not being completely honest with her mother when Juliet tells her that she does not want to marry.
A second explanation can be that Shakespeare captured the mind of a girl Juliet's age perfectly. Due to limited experiences, a person who is very young will often change their mind in a matter of a heartbeat. So while Juliet may have woken up that morning being completely uninterested in men, love, and marriage, it is only because she had not yet met a man she thought was handsome, charming, and seductive. Hence, after meeting Romeo and being not only flirted with for the first time but even actually kissed for the first time, it is perfectly natural for Juliet to go to bed that night with her hormones buzzing and full of amorous thoughts and feelings. It is perfectly natural for her to proclaim after meeting him, "If he be married, / My grave is like to be my wedding bed" (I.v.143-44), despite what she told her mother earlier that day.
Hence, we can conclude that we see Juliet have different reactions towards love either because she is not completely honest about her thoughts and feelings with her mother or because she is just too young to truly know what she wants and does not want.