We can see why Romeo speaks of love when we first meet him in the opening scene. Romeo actually equates love with lust, rather than any real feelings of commitment and devotion. Romeo tells Benvolio that he is blue, not only because Rosaline does not return his affection, as we see in the line, "Out of her favour, where I am in love," but more importantly because Rosaline refuses to let him take her "maidenhead." We see this in Romeo's line,
... she hath Dian's wit;
And, in strong proof of chastity well arm'd,
From love's weak childish bow she lives unharm'd.
Diane is a Greek goddess known for her chastity, hence Romeo is saying that Rosaline has refused to sleep with him. Therefore, Romeo is not just sad because Rosaline won't return his affection, he is sad because Rosaline won't engage in sexual activity with him. To Romeo, feelings of love and feelings of lust are the exact same thing, so it is not surprising when in the balcony scene Romeo talks passionately about Juliet's beauty and equates his admiration of her beauty with feelings of love. Romeo speaks only of love in this scene, because love and lust are the only things on his mind.
Juliet, on the other hand, does equate feelings of love with commitment. Her mother has already made it clear to Juliet in Act 1, Scene 3 that it is her duty to marry, and that she is expected to marry Paris. Even though Juliet tells her mother that marriage is "an honour that I dream not of," her first thought after meeting Romeo is that, "If he be married, / My grave is like to be my wedding bed" (Act 1, Scene 5). In other words, even though she tells her mother that she does not wish to marry, her first thoughts are of marrying Romeo. This proves that she equates love with commitment and marriage.