I am going to assume that your question concerns Act I, scene i, as Romeo and Benvolio are discussing why Romeo is moping about. Benvolio, just before Romeo's entrance, has been discussing with his parents their concern over the cause of Romeo's "heaviness." Lord Montague describes Romeo in this way:
Away from light steals home my heavy son
And private in his chamber pens himself,
Shuts up his windows, locks fair daylight out. . .
Black and portentous must this humour prove
Unless good counsel may the cause remove.
So, Benvolio is elected to try to find out why Romeo is so heavy-hearted. He finds that the answer is that Romeo is in love with a girl who does not return his affections. This state of affairs is the reason for Romeo's melancholy view of love. He calls love
. . .a smoke made with the fume of sighs;
Being purg'd, a fire sparkling in lovers' eyes;
Being vex'd, a sea nourish'd with lovers' tears;
What is it else? A madness most discreet,
A choking gall, and a preserving sweet.
For Romeo, as the above lines indicate, because he loves (Yea!), but his love is not returned by Rosaline (Boo!), he sees it as simultaneously wonderful and horrible, as a "feather of lead, bright smoke, cold fire" and "sick health."
So, to answer your question, it is the fact that Romeo's love is not returned that causes him to see it in such an extreme and contradictory fashion.
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