The main purpose of this sentence is to alert "city people" to forget their expectations and yearning for pure, natural and beautiful things, because they follow the rules of social life. It's a bit abstract, but you will understand it after careful taste, and there is the last paragraph of the full text, "It's just that birds and my son never get curious about it." It is to sublimate "city people", a dull attitude towards life in today's society, materialistic and boring, forgetting human nature. On the other hand, it also reflects the helplessness of the "father" in the heavy social life.

Welcome to discuss. -Teacher Tang