Answer: At its core, Shakespeare's famous tragedy is a story about the all-consuming power of love. There are several love themes in Romeo and Juliet, but the strongest is how powerful love can be. We first see its depths when Romeo laments being out of favor of Rosaline, his first love:
"ROMEO
Why such is love’s transgression.
Griefs of mine own lie heavy in my breast,
Which thou wilt propagate to have it prest
With more of thine. This love that thou hast shown
Doth add more grief to too much of mine own.
Love is a smoke made with the fume of sighs;
Being purg’d, a fire sparkling in lovers’ eyes;
Being vex’d, a sea nourish’d with lovers’ tears:
What is it else? A madness most discreet,
A choking gall, and a preserving sweet."
- Act 1, Scene 1
Romeo's love for Rosaline, and his heartbreak at her disaffection, affects everything he says or thinks. The only force strong enough to break him of this spell is his newfound love for Juliet, which occurs at full force for Romeo upon seeing her:
"ROMEO
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
As a rich jewel in an Ethiop’s ear;
Beauty too rich for use, for earth too dear!
So shows a snowy dove trooping with crows
As yonder lady o’er her fellows shows.
The measure done, I’ll watch her place of stand,
And touching hers, make blessed my rude hand.
Did my heart love till now? Forswear it, sight!
For I ne’er saw true beauty till this night."
- Act 1, Scene 5
The love between Romeo and Juliet drives the plot forward, as nearly every decision they make is in service of their love for each other. One such decision is their choice to marry each other the day after meeting:
"ROMEO
"Ah, Juliet, if the measure of thy joy
Be heap’d like mine, and that thy skill be more
To blazon it, then sweeten with thy breath
This neighbour air, and let rich music’s tongue
Unfold the imagin’d happiness that both
Receive in either by this dear encounter.
JULIET
"Conceit more rich in matter than in words,
Brags of his substance, not of ornament.
They are but beggars that can count their worth;
But my true love is grown to such excess,
I cannot sum up sum of half my wealth."
- Act 2, Scene 6
Romeo's decision to stop the duel between Mercutio and Tybalt is another example of his love for Juliet. But when Tybalt kills Mercutio anyway, Romeo's love for his friend blinds him — just as his love for both Rosaline and Juliet blinded him before — and he kills Tybalt in an act of rage. The literary theme of love is responsible for both the happiness and the tragedy in the play.