This characteristic (of 100% loyalty) is actually her tragic flaw.  Juliet is so loyal to Romeo and her love for him, that she will not leave him.  Her parents want her to marry Paris (but are completely ignorant to her being already married).  And after getting yelled at by Lord Capulet, the Nurse even tries talking Juliet into marrying Paris.  However, Juliet will not leave her man.  She is loyal to him until the end.  When he takes his life (because he thinks that she is dead, too), she loyally takes her own life.  She cannot live without him and will not live without him.  This just solidifies her true tragic flaw.  That is why it's such an important point in the play.