Juliet's rebellion begins for several reasons. First is her lack of decision about her own life. Before we hear a word from Juliet herself, her fate and future are already being plotted by her father, who intends her to marry Paris (a gentleman, the marriage to whom would benefit the family both politically and economically). Lord Capulet may claim to have Juliet's interests at heart, and while he encourages Paris to be kind, the truth lies in this declaration: "My will to her consent is but a part / And she agreed, within her scope of choice / Lies my consent and fair according voice" (1.2.16-18).
While Juliet may have consented initially, her sheltered life had not afforded the young girl many glimpses of other choices. When she sees Romeo at the ball, she falls in love immediately. Her father's selection of a mate no longer holds much import.
Another factor, correlative to this one is the biology of her age. Juliet is a teenager; her hormones are raging. Many a parent can attest to how their once obedient child became much less so during their teenage years.
Still another reason is Juliet's lack of parenting from her mother and a nurse whom she has bent around her little finger. Juliet has no strong adult to turn to as she makes her fateful decisions. All of these reasons, and more, are instrumental in changing Juliet's desire to please her parents.