Even though Romeo and Juliet are happily married and content, Shakespeare visibly implies that their bliss will be fleeting.  Throughout the first two acts of Romeo and Juliet, Shakespeare uses foreshadowing very well to convey to the reader that the happiness and comedy displayed in the first two acts will soon turn to doom.  The most likely or important threat, I think that will interfere with Romeo and Juliet's happiness is fate.

For example, throughout Romeo and Juliet, stars are a symbol often referred to describe fate.  It was commonly believed in that time, that the stars controlled people's lives by fate or destiny (margin).  In the prologue is one of the most important signs that foreshadow the death of the two lovers.  "A pair of star-crossed lovers take their life... the fearful passage of their death-marked love...