The love being divided is one theme, as symbolized through night and day. Night brings the two lovers together, but since Romeo has been sentenced to death if he does not leave Verona by morning, morning separates them. Not only has morning separated them, but circumstances that have stemmed ultimately from family choices have also separated them, such as Tybalt's death.
Impending death is also a theme. After Romeo descends to the garden from the window, Juliet says:
I have an ill-divining soul!
Methinks I see thee, now thou art below,
As one dead in the bottom of a tomb. (lines 43-45)
This statement serves to foreshadow both Romeo's and Juliet's upcoming deaths.
A third theme is filial obedience. Juliet's parents believe it is Juliet's duty to obey their wishes, especially her father's. However, in refusing to marry Paris, Juliet is disobeying them and because of this they think she is ungrateful for their upbringing and want nothing further to do with her.
Likewise, a fourth theme that comes into play is the rightfulness of parents' decisions and even of their tempers. Lord Capulet raises an excellent point that he has found a noble and fine match for his daughter to marry. However, he should question whether or not she is truly ready to marry and go instead with his initial instinct that Juliet should wait two more years. Lord Capulet's temper is also questioned when his wife declares, "You are too hot," proving that one major theme in this scene is whether or not parents act rightly.
Several themes in Romeo and Juliet could be:
Love will find a way
True love knows no boundaries
Do not be afraid to change those around you
Think before you act
Passionate thinking is like that of a drunkard