says this line in Act I, before he has met Juliet
. The subject of the speech it is part of, copied below, is what we might call the "upsides" and the "downsides" of love. Romeo here understands love as both full of pain and full of joy: pain when love goes badly, joy when it goes well.
On the downside, love is compared to smoke--that which blinds and stings the eyes. This downside occurs when we are "sighing"--longing for--the beloved. The "smoke" is created from our sighs. Shakespeare is likening the breath of sighing to smoke (breath, especially on a cold day, which would be a day when we are deprived of our lover's warmth, does look like very much like smoke). However, when the longing is "purged" by the beloved's presence, we're back on the upside, and love goes from painful, ugly smoke to the positive image of "a fire sparkling in the lovers' eyes." And of course, it would be a fire, as we are warm in a lover's presence.
Shakespeare then goes back to the downside, that when we are "vexed," or thwarted, annoyed, by love, it is a sea nourished by tears (playing on the sea being salt water) or something bitter tasting ("gall") that we choke on but also a "sweet" (dessert) that preserves (with a play on food being preserved) us. Romeo understands love as a form of madness, probably because of the way it jolts our emotions, making them careen wild from joy to despair and back again. As he puts it:
Love is a smoke raised with the fume of sighs;
Being purged, a fire sparkling in lovers' eyes;
Being vexed, a sea nourished with loving tears.
What is it else? A madness most discreet,
A choking gall, and a preserving sweet.