It is clear from the text that Juliet was mostly raised by the nurse. This fact becomes evident in Lady Capulet's conversation with Juliet's nurse in Act One, scene three. The nurse recalls her memories of Juliet since the child's birth, to the present, so much so that Lady Capulet (probably in a fit of envy) calls her to order. The nurse cannot, however, stop prattling--an indication of her devotion to Juliet.
It was common practice (and still is in many societies today) for a nurse or nurses to take care of a child or children from birth, especially in well-to-do families. This involved not only taking care of the physical aspects, such as dressing and feeding, but could also include the most basic parts of a child's upbringing, such as educating the child about values, rules, etc. that the family wished to inculcate.
Obviously, the parents instructed the nurse about what exactly her rights and responsibilities were, for they were the employers and of course, the child's parents. This, however, did not stop the nurse from developing a deep personal relationship with the child in her care and it often resulted in a very strong bond between the two. It has often been found that the nurse knows more about the child in every way than the parents ever could. This did not mean that the child would become entirely alienated from the parents since they were made completely aware of exactly who their fathers and mothers were from an early age. Parents exercised control over the child's freedom and set limits to his/her actions.
One could invariably find that because of the close bond shared by the child and nurse, it would become easier for the child to confide in his/her 'nanny', as she is commonly called, and the nurse would be more than willing to cater to his/her needs. This is/was especially true if the child rebelled against the parents' instructions. This is true of Juliet and the nurse in the play. Juliet uses her as a go-between in her secret affair with Romeo. The nurse runs errands for her and Juliet truly trusts her. Her trust is never unfounded because the nurse does not betray her, but grants Juliet her every whim.