One of the themes in the film is the struggle of three princesses who are to be married off to distant princes to aid political relationship between the kingdoms.  Throughout the film, the three princesses (and their adoring guards) become inspired by Mulan’s own philosophy:

My duty is to my heart.

Mulan shared this when asked whether she knew it was the right thing to listen to the customs of her time that said it was not allowed for women to leave home to join the military or whether she should listen to her heart, which told her to do so.

In the end, we have a responsibility to our social customs and graces, but the true guiding principle in our life is our conscious.