The speaker in sonnet 18 by petrarch uses negative words and phrases throughout the sonnet to express the fact that

The speaker in sonnet 18 by Petrarch uses negative words and phrases throughout the sonnet to express the fact that he was unable to capture the his beloved's beauty in the verse. 

The speaker in Sonnet 18 by Petrarch uses negative words and phrases throughout the sonnet to express the fact that he was unable to put in words how gorgeous his beloved one was.

During the time he wrote this sonnet he was passing by a writers' block and he was incapable to show his beloved Laura how beautiful he thought she was by words so he repeats in many ways that there's nothing that can describe, capture or equal her beauty.

This happens because our cells used up the potential energy which is stored inside the glucose and losing that energy during exercise reduce our mass.
Glucose is composed of chemical energy which are used by the cells to survive. When the glucose molecule is burn up, energy is always release.