In William Shakespeare's play Romeo and Juliet, the two young lovers were born into the Montague and Capulet families, which were involved in a long and vicious feud. The main obstacle to a happy romantic ending to the play is that the families are both vehemently opposed to the marriage.
The Chorus, speaking the Prologue to the play, attributed the tragedy to the ancient feud between the two houses, taking an almost classically tragic approach (similar to choruses in Oedipus Rex or the Oresteia) to the theme of bloody feuds engendering more violence. This viewpoint is articulated in the lines:
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life;
The phrase "star-crossed" has astrological import and suggests that they are doomed by the very configuration of the stars at their births.
Juliet's famous speech in which she asks "wherefore art thou Romeo?" also notes that the main obstacle to their love is that he was born into the Montague family and that she is a Capulet.