What is the overall theme of Romeo and Juliet?

One of the main themes of Romeo and Juliet is that actions have unintended consequences.

Whatever the cause initially of the feud between the Montagues and the Capulets, it obviously got out of control.  Much blood was shed for many years, and it is clear that when the play starts no one really remembers the initial conflict.  They are just killing each other with impunity, because the two families hate each other.  Shakespeare clearly tells us in the prologue that this feud is old.

Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean. (Act 1, Prologue)

Another perhaps inevitable unintended consequence of the feud was Romeo and Juliet falling in love.  You see, when something is forbidden it becomes all the more attractive.  If Juliet had not been a Capulet, Romeo would not have met her at the ball, and he would not have been there in secret. Romeo and Juliet had never met because their families were feuding.  Yet the feud gave neither of them much pause.

My only love sprung from my only hate!
Too early seen unknown, and known too late!
Prodigious birth of love it is to me,
That I must love a loathed enemy. (Act 1, Scene 5)

Romeo and Juliet’s love seems innocent enough, other than the silly fighting between their families.  However, it causes a snowball effect of unintended consequences.  Because Tybalt is angry at seeing Romeo at the ball, he attempts to fight him.  Romeo tries to help his friend Mercutio, who fights Tybalt instead of Romeo, and in doing this Romeo accidentally causes Tybalt to impale Mercutio.

A plague o'
both your houses! 'Zounds, a dog, a rat, a mouse, a
cat, to scratch a man to death! a braggart, a
rogue, a villain, that fights by the book of
arithmetic! Why the devil came you between us? I
was hurt under your arm. (Act 3, Scene 1)

Another unintended consequence of Romeo not fighting is that once Tybalt kills Mercutio, Romeo has to fight him.  He kills Tybalt, and gets banished for it.

That leads us to the greatest unintended consequence, Romeo and Juliet’s death.  To try to be with Romeo, Juliet fakes her death.  Romeo thinks that she is dead and kills himself, and then she wakes up to find him dead and kills herself.

Interestingly enough, this leads to our last unintended consequence.

A pair of star-cross'd lovers take their life;
Whose misadventured piteous overthrows
Do with their death bury their parents' strife. (Act 1, Prologue)

Of course, the parents would probably prefer that it didn’t take the deaths of their children to end the feud, but at least something good came from so much misery.

The theme of unintended consequences runs like a thread through the play.  Love is messy, Shakespeare is telling us.  We often make a choice, and that forces other choices.  In the end, Romeo and Juliet make bad choices, and their parents make bad choices.  Mercutio, Tybalt, and Friar Laurence make bad choices.  Yet in the end, at least a good choice is made for the two families to bury their hate and live peacefully together.