Explain how how Shakespeare presents the relationship between Romeo and Juliet in the conversation in act 1, scene 5 and in the play as a whole.

In act 1, scene 5 of Romeo and Juliet, we see our first glimpse of the ill-fated couple together. In the scene, often called the party scene, Romeo and his friends crash a party at the Capulets's house. Romeo, in love with Rosaline at this point, is looking for her at the party. Juliet, on the other hand, is merely hoping to have fun and dance.
The relationship between the two is presented as almost a foreshadowing of what is to come. Shakespeare carefully crafts the way that they interact to demonstrate a common theme that runs throughout the play—love that burns quickly also extinguishes quickly. Romeo and Juliet experience a moment of love at first sight—Romeo is so taken with Juliet that he forgets about Rosaline altogether. The suddenness of their meeting, talking, and kiss is a precursor to show the quick nature of their love—something that is destined to end quickly. Shakespeare makes a point in the play that any relationship that moves that quickly is destined to dissolve quickly, whether by misfortune or death.
Romeo and Juliet are considered “star-crossed” lovers, meaning that their paths are not meant to meet, and when they do, only disaster can result. Friar Lawrence says that their relationship is like “fire and powder” and that “these violent delights have violent ends”—things that move so quickly also move toward their end just as quickly.
In act 1, scene 5, Shakespeare establishes the quickness of their relationship by having Romeo instantly fall in love with Juliet—he has Romeo reflect on her beauty from afar when he says,

The measure done, I'll watch her place of stand,
And, touching hers, make blessed my rude hand.
Did my heart love till now? forswear it, sight!
For I ne'er saw true beauty till this night.

Romeo, until this point in love with Rosaline, questions if he had ever even seen beauty until he set eyes on Juliet. That lightning-quick change in his feelings shows the dangerous nature of their love: anything that strong that quickly is not built to endure. Like a fire that bursts quickly and burns hot, their passion is unlikely to last very long.
The reason for the quick burn, in this case, we find out at the end of the scene after the two have exchanged flirtatious dialogue and kissed. Romeo and Juliet both find out that their families are mortal enemies as they are leaving the party, and that is part of the reason their love is fated not to endure—they had to fall in love quickly because if they had known each other’s names, it is unlikely they would have ever spoken.
The temporary nature of their love and relationship is portrayed in this first scene, setting up the tragedy of the rest of the play. We, as an audience, are meant to empathize with their intense romance and the grand romantic gesture of suicide at the end of the play. Shakespeare sets it up so there is almost no other way this story could have ended: if Romeo and Juliet had lived, it is likely they would have outlived the passionate love they found at the party that night.