"sad old earth..."
See in text (Solitude)
This is an example of personification, or attributing human qualities to nonhuman things. Personification serves to make things, like the earth, more sympathetic to humans, which can make it easier to form comparisons to seek understanding of the human condition. Here, the speaker attributes the earth with “sad” and “old” qualities and an ability to act with agency. (It “must borrow its mirth.”) By doing this, the speaker draws the earth into the human realm, portraying it as a melancholic entity that needs to draw on the happiness of others. This appears to add weight to the positive assertions that the speaker makes, emphasizing the ability of one’s actions to positively affect others.