What is the subject matter of Romeo and Juliet?

By "subject matter" I think that you are asking about what the play's universal themes might be.  

One major theme of Romeo and Juliet is the theme of love.  Shakespeare presents a few aspects of love during the play as well.  Yes, absolutely, the play is full of romantic love.  Romeo and Juliet are constantly gushing about what the other person means to them and explain in flowery language how beautiful the other person is.  The play also explores the notion that death and dying for love is better than living without your true love.  Winnie from Tuck Everlasting would disagree.  She chose to live without Jesse Tuck, but I digress.  

But the play does more than explore the love between Juliet and Romeo (or Romeo and Rosaline).  Their is no doubt that Shakespeare also wrote about parental love.  Juliet's parents clearly love her and want the best for her.  Lord Capulet clearly respects Juliet's feelings and wants her to marry somebody that she likes.  That's why he tells Paris to wait until Juliet is older.  Lady Capulet is the same way.  She believes that Paris is a good match for Juliet, but Lady Capulet wants Juliet to see that too.  That's why before the party Lady Capulet tells Juliet to keep an open mind about Paris.  

Romeo and Juliet is also full of revenge and hatred themes.  The Capulets and Montagues hate each other.  What's important for the reader to note is that the feud has been going on for a long time, and nobody seems to remember what initially started it.  The two families just go about hating each other.  Then, once Tybalt kills Mercutio, the rest of the killings follow an eye for an eye revenge motif.  Romeo kill Tybalt, which pits the entire Capulet family against Romeo.  It even pits Paris, a man who hopes to be part of the Capulet family, against Romeo.