Answer: The two reasons the speaker gives for why all fair things decline are: nature and chance.
Step-by-step explanation:
In Shakespeare's Sonnet 18 (1609), the speaker discusses whether he should compare a young man to a summer day. He realizes that no such comparison should be made, as youth's beauty surpasses a summer day. Among the reasons why he considers a young man more lovely than a summer's day, the speaker lists that everything beautiful will eventually stop being beautiful, either by chance or in the course of nature:
"And every fair from fair sometime declines,
By chance or nature’s changing course untrimmed."
However, the beauty of youth never fades. The young man will, therefore, live forever in this poem, and remain as beautiful as he is now.