In the Prologue to Act I of Romeo and Juliet the Chorus states,
A pair of star=cross'd lovers take their life;
Who misadventured piteous overthrows
Do with their death bury their parents' strife. (1.1.6-7)
The marriage of Romeo to Juliet is the second of their "misadventured piteous overthrows"--a series of events that keep them apart. After the words of Friar Laurence,
these violent delights have violent ends,
And, in their triumph die, like fire and powder,
Which as they kiss, consume. (2.6.9-11)
Romeo commits his second act of "misadventured overthrow"--the first is invasion of the party and meeting with Juliet--as he encounters Tybalt and seeks to ameliorate the tense conflict between Tybalt and Mercutio. His well-meaning declaration of love for Tybalt now that he is related to the Capulets through marriage is misunderstood and backfires as it causes Tybalt to become so enraged that he stabs Mercutio under Romeo's arm.
As a result of his friend's angry death and curse upon "both your houses," Romeo loses his dear friend, whom he has tried to save from harm, Romeo's act of love for Tybalt becomes overthrown by the insidious act of Tybalt's having stabbed Mercutio and Romeo having, then, killed Tybalt--"a piteous overthrow," indeed.