The word "stars" is certainly used as a recurring motif to refer to Romeo and Juliet's love. In fact, we first see the term in the Prologue when the chorus addresses Romeo and Juliet as "a pair of star-cross'd lovers" who "take their" lives (6). The term "star-crossed" is a way of predicting future doom. The lovers are being called ill-fated, which means that fate has decreed they shall never be united and never be happy, but instead die. Hence, the recurring motif of the "stars" is a way for Shakespeare to allude to the couple's upcoming peril that their love is leading to.
The word "stars" is also used as a recurring motif to allude to romance, especially the couple's romance. Capulet encourages Paris to court Juliet, inviting him to join their feast in the lines:
At my poor house look to behold this night
Earth-treading stars that make dark heaven light:
Such comfort as do lusty young men feel
When well-apparell'd April on the heel....(I.ii.24-27)
Capulet's reference to "stars" at night in conjunction with "lust" and "April" shows us that he is using "stars" to refer to nighttime and all the romantic activity that takes place at night. Hence, stars is also being used as a motif to refer to romance and the ill-fate that the two lovers will soon fall prey to.
Likewise, Romeo compares Juliet's eyes to stars in the famous balcony scene, referring again to nighttime and indirectly alluding to their star-crossed love. Romeo compares her eyes to "two of the fairest stars in heaven" because he is looking at her at nighttime, but also because he is thinking of her romantically and even lustfully (II.ii.15). Hence, even in this passage, "stars" refers to the couple's upcoming romantic activity, but also to their upcoming deaths.