How is the theme of love in the play Romeo and Juliet still relevant in today's society?

Romeo and Juliet seems intended to demonstrate it is only in youth that people are willing to give their hearts so completely to another. No doubt they are destined to be disillusioned, but perhaps the experience is worth the suffering that goes with it.

When I was one-and-twenty
      I heard a wise man say,
“Give crowns and pounds and guineas
      But not your heart away;
Give pearls away and rubies
      But keep your fancy free.”
But I was one-and-twenty,
      No use to talk to me.

When I was one-and-twenty
      I heard him say again,
“The heart out of the bosom
      Was never given in vain;
’Tis paid with sighs a plenty
      And sold for endless rue.”
And I am two-and-twenty,
      And oh, ’tis true, ’tis true.

      A. E. Housman, A Shropshire Lad

Later on in life people become more cautious about other people and commitments. They have learned by experience that everything changes, nothing lasts. King Claudius tells Laertes in Hamlet:

There lives within the very flame of love
A kind of wick or snuff that will abate it.  
                       Hamlet, Act IV, Scene 7

Claudius is a middle-aged man, and his and Gertrude's love for each other is a relatively tepid, middle-aged love. Romeo and Juliet's sudden love is "that first fine careless rapture" which is so strong they are willing to die for each other. That kind of love is not rare. Young people still experience it today. It is one of the main joys and sorrows of adolescence, and it is largely because of their youth and inexperience that they can indulge in it so unreservedly. Older people who read Romeo and Juliet or see it performed have two-dimensional feelings about it. They know Romeo and Juliet's love can't last. They know it is synonymous with youth. They know it is beautiful while it lasts, just as youth itself is beautiful while it lasts. 

Pyotr Ilyich Tchaikovsky wrote a famous overture-fantasy about Shakespeare's Romeo and Juliet. There is a love theme in that music which seems to capture the mood Shakespeare was trying to convey in words. There are five notes which go higher and higher up the scale and in volume until it seems as if the love those notes represent will soar into the stratosphere. The music is beautiful. The play is beautiful. The human love is beautiful — while it lasts. It was inevitable that Romeo and Juliet's relationship would end, either by death or because their love grew cold. The tragic aspect of Shakespeare's Romeo and Juliet is intertwined with — and just as essential to the story as — the love story.