How does Shakespeare present or show the pain of love in Romeo and Juliet?

There are lots of quotes in Romeo and Juliet that show the pain of love.

We know the pain of love is a main theme of this play because it is first mentioned in the chorus: "The fearful passage of their death-mark'd love ... Is now the two hours' traffic of our stage." This means that the horrible story of Romeo and Juliet's doomed love is the topic of the play.

In Act I scene I, Romeo pines for Rosaline and shares his sadness with his cousin, Benvolio. When they come across the site of the Act I scene I fight between Capulet and Montague servants, Romeo highlights the paradox of love and hate in the same line, and then repeats this paradox: "Here's much to do with hate, but more with love. / Why, then, O brawling love, O loving hate," shows the audience how closely related the two feelings are. "Brawling" means fighting, so juxtaposing the word "brawling" against the word "love" could mean that love leads to fighting and hatred, or that both love and hate lead to inner conflict.

A few lines later, Romeo describes the feelings of being in love using metaphors: "Love is a smoke made with the fume of sighs." Smoke darkens the sky and makes you choke; fume could be a poisonous or stinky gas, and we sigh when we are sad or melancholy. Smoke, fume, and sighs are all gasses -- intangible, impossible to hold, formless. So Romeo is saying that love has these qualities: love is formless, impossible to hold, it makes you choke, darkens your sky, and makes you feel melancholy.

In Act I scene IV, while Romeo, Benvolio, and their friend Mercutio are heading to the Capulet party, Romeo says more quotes that show the pain of love. He uses another metaphor: "Under love's heavy burden do I sink," which makes love sound like about as much fun as a pocket full of rocks pulling him down under water. Then he gives human qualities to love in the personifications: "Is love a tender thing? It is too rough, / Too rude, too boisterous, and it pricks like thorn." Here Romeo is saying love is rude and argumentative. With the simile "pricks like a thorn" he compares love to a rose, but not the beautiful rose flower people usually associate with love. Instead Romeo thinks about the thorns: the part of the rose plant that might hurt you if you try to pick a rose for your beloved. This seems to indicate that love seems sweet and tender, but hiding just beneath is the potential for a lot of pain.