In Act I, Romeo is pining over Rosaline and says:
Love is a smoke raised with the fume of sighs;
Being purged, 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.
That is, love is as ethereal as a smoke, which slips our grasp, but our sighs (when we are in love) create this smoke. When the smoke is gone, love is the fire you see in your lover's eyes. When love is frustrated--which is the case with Romeo here, since Rosaline won't give him the time of day--it provokes a sea of tears. Love is a madness. It's paradoxical in that it's something you both desire (a sweet) and something that chokes you on its bitterness.
Later, Juliet, speaking with Romeo, says:
My bounty (of love) is as boundless as the sea,
My love as deep; the more I give to thee,
The more I have, for both are infinite.
'Tis the paradoxical nature of love that the more you have, the more you give away and the more you give away, the more capacity for love you have.