Links to themes:
Love and Hate
The love between Romeo and Juliet symbolizes life, for it is elevating, and brings hope (even if it is for a short period of time) to them.
The hatred the two different households have for each other symbolizes death, for it is destructive, and eventually brought about the deaths of four people.
Reason and Passion
Most of the major events that happened (and led to the tragic deaths) wouldn't have occurred, if not for the passion of the characters. E.g. Tybalt's passion in upholding family honour which led to Mercutio's death and his own, and Romeo's marriage to Juliet.
Fate and Freewill
It is Fate that set the events and coincidences for the eventual deaths of Romeo and Juliet. E.g. The children of the two feuding households would fall in love with each other.
It is the character's Free Will, that led them to do certain undesirable actions, which contributed to the tragic end of the play. E.g. It was Mercutio's decision to fight Tybalt in order to uphold Romeo's honour that led to his own death.
Age and Youth
The old did not understand or appreciate the needs of the young, and this formed an unbridgeable gap between them, which eventually led to the deaths of the characters. E.g. Lord Capulet did not understand Juliet's need to marry for love, and he forced her to marry Paris, hence resulting in her faking her "death". This became the buildup to the ending, where both Juliet and Romeo died.
Order and Disorder
The disorder created by the Montagues and Capulets (feud) were ended by the deaths of Romeo and Juliet. Their sacrifices resulted in eventual order and peace between the two households.