This love that thou hast shown
Doth add more grief to too much of mine own.
Love is a smoke raised with the fume of sighs;

This quote shows that is often combined with "grief." Opening up to love someone means also opening up to the grief of losing them. Love is also connected to smoke and fumes, bringing up the concept of getting burned by fire.

My only love sprung from my only hate!
Too early seen unknown, and known too late!
Prodigious birth of love it is to me,
That I must love a loathed enemy.

This quote shows the complexity of love. Love and hate often go together. To love someone who is supposed to be an enemy creates so many problems for Romeo and for Juliet and their families.

It is the east, and Juliet is the sun!—
Arise, fair sun, and kill the envious moon,
Who is already sick and pale with grief,
That thou her maid art far more fair than she.

Here love is once again being discussed alongside grief and sadness. This quote is showing that, even when seeing Juliet, in all her beauty and brightness, Romeo has thoughts of death (kill the envious moon) and sadness (pale with grief).

This quote illustrates the bad that can come even from something good, like love.