What things in life made the narrator sad?

From the text:

Each person has what he doesn’t want, and other people have what he does want. Married men have wives, and don’t seem to want them; and young single fellows cry out that they can’t get them. Poor people who can hardly keep themselves have eight hearty children. Rich old couples, with no one to leave their money to, die childless. Then there are girls with lovers. The girls that have lovers never want them. They say they would rather be without them, that they bother them, and why don’t they go and make love to Miss Smith and Miss Brown, who are plain and elderly, and haven’t got any lovers? They themselves don’t want lovers. They never mean to marry. It does not do to dwell on these things; it makes one so sad.