The fact that the couple secretly marries without benefit of Juliet's father's permission hints to us that something will go wrong because the rules for marriage have been broken. Paris follows these rules, but Romeo breaks them. The Friar's agreement to marry them is a huge faux pas on his part as well.
The impetuousity on the part of the Friar, Romeo, and Juliet to marry foreshadows the later rash decisions of Romeo and Juliet to immediately kill themselves without thinking it through. Even the Friar's rashness in marrying the couple foreshadows his act later on of making the potion for Juliet to get her out of the marriage to Paris.
Of course, the fact that Romeo and Juliet are the only children of feuding families foreshadows that this cannot end well. Tybalt became very angry with Romeo at the party, and we know that his hot temper won't leave this alone. The marriage further complicates this situation, foreshadowing the duel to come later between Tybalt and Romeo.
Although the marriage about to take place in Act 2, scene 6 is a peaceful scene, it does contain elements that foreshadow future sorrow. First, the Friar, upon blessing the marriage, asks the heavens to smile on the couple no matter what hardships may befall them. This is a common wish at a wedding, yet Romeo accepts his statement with bravado. He challenges "love-devouring death" to "do what he dare." Romeo believes that if he is with Juliet, all will be fine. Of course, love-devouring death rises to Romeo's challenge. When Friar Lawrence scolds Romeo for his ego, his words also foreshadow the tragedy to follow. He says, "These violent delights have violent ends/ And in their triumph die, like fire and powder,/ Which as they kiss, consume." This basically means that only bad can come from having too much good. Therefore, he advices Romeo to "love moderately." Despite his good advice, however, the Friar still marries the couple that afternoon.
Because the audience in the elizabethan era and now were told in the prologue that Romeo and Juliet are star-crossed lovers. The "crossed" represents death and the stars are what people in the elizabethan era belived to be almighty.