Here sorrow is spoken of as if it were a person that could follow someone else. AT: "the one who is alone when he falls is sorrowful" (See: [[rc://en/ta/man/translate/figs-personification]])
The writer speaks of two people keeping each other warm on a cold night. AT: "If two people lie down together at night, they can be warm" (See: [[rc://en/ta/man/translate/figs-explicit]])
# how can one be warm alone?
This refers to a person lying down. The author use this rhetorical question to emphasize that two people can keep each other warm but one person cannot. This question can be written as a statement. AT: "a person cannot be warm when he is alone." or "a person who lies down alone cannot be warm." (See: [[rc://en/ta/man/translate/figs-rquestion]] and [[rc://en/ta/man/translate/figs-ellipsis]])