The term "star-crossed" refers to astrology, which was a prevalent belief system in Shakespeare's day. The play contains a great deal of imagery related to the celestial bodies, particularly in lines spoken by Romeo and Juliet in their conversations with one another. ("Oh swear not by the moon, the inconstant moon!"; "What light through yonder window breaks? It is the east, and Juliet is the sun." "And when he should die, take him and cut him out in little stars.")
Their intense attraction inspires a feeling of fate and rightness, as if they were destined to meet and fall in love. But their fates are pre-determined to some extent by the family conflict between the Capulets and Montagues. The lovers have to hide their relationship, and meet under cover of night, further underscoring the theme of how the "stars" rule their destiny. Their final desperate meeting is fraught with miscommunication and chaotic misunderstanding, and such events are said to occur under in a period of retrograde motion of the planet Mercury, which rules travel and communications. The message is that despite the best intentions of humans, their actions may be influenced by forces beyond their control. Since Romeo and Juliet were doomed from the start, no amount of rebelling against their fate would have changed anything.