Question

Why does the speaker in sonnet 18 by petrarch repeatedly use negative words and phrases?

Answer 1

The poem is about his inability to spend time with his beloved. 

The poem is about his inability to capture his beloved's beauty in verse. <-- 

The poem is about his inability to accept that his beloved has died. 

The poem is about his inability to understand what others see in his beloved. 

Answer 2

Petrarch's Sonnet 18 is about Laura, her beauty and his incapability to describe her beauty in a proper way. His love for her is an impossible one, and he uses repeatedly negative words and phrases to state clearly that her beauty is impossible for him to describe, "unsung...in my rhyme".  He hasn't got any possibilities to make a proper description of her beauty, nor to describe her brightness. So he uses negative word and phrases to strenghten  the idea of his inability to make a suitable portrayal of her, because her beauty is such that it exceeds his chance to describe it. He hasn't got the strength nor the genius.