Why did Romeo and Juliet kiss all the time?

In Act I, scene v, of "Romeo and Juliet", the two characters meet at the party at the Capulet's house.  This is the only scene where Shakespeare actually includes a "kiss" as a part of the action of the plot.  The kiss is part of a series of flirtatious "pick up" lines that they use on one another.  Here is the scene:

ROMEO
[To JULIET] If I profane with my unworthiest hand
This holy shrine, the gentle fine is this:
My lips, two blushing pilgrims, ready stand
To smooth that rough touch with a tender kiss.

JULIET
Good pilgrim, you do wrong your hand too much,
Which mannerly devotion shows in this;
For saints have hands that pilgrims' hands do touch,
And palm to palm is holy palmers' kiss.

ROMEO
Have not saints lips, and holy palmers too?

JULIET
Ay, pilgrim, lips that they must use in prayer.

ROMEO
O, then, dear saint, let lips do what hands do;
They pray, grant thou, lest faith turn to despair.

JULIET
Saints do not move, though grant for prayers' sake.

ROMEO
Then move not, while my prayer's effect I take.
Thus from my lips, by yours, my sin is purged.

JULIET
Then have my lips the sin that they have took.

ROMEO
Sin from thy lips? O trespass sweetly urged!
Give me my sin again.

Romeo first implies that by touching her with his hand, he might have been wrong - all because she is too worthy to be touched.  He offers to kiss the place that he touched in order to make up for it.  Juliet is coy and responds that his hand is plenty worthy, and that lips should be used in prayer.  Romeo keeps pressing on to say that he is a sinner, and must kiss her lips to get rid of his sin - which he does.  Then Juliet teases him into kissing her again by suggesting that he has given his sin to her, and must take it back again.

Shakespeare is just showing us two teenagers at a party, behaving as teenagers do - flirting and kissing.  By having Juliet request the kiss back, we can see that she is just as interested as he is, and that she is bold enough to be that forward.