One of the meanings that this metaphor could have is that of "being" in life. The metaphor could be comparing the way fish are to the way a man lives his life. Fish simply exist, the "are" in life, and they do not worry about having a meaning, a purpose, a goal or a duty. This could be applied to the way a man should live his life. Instead of looking for a meaning or for something external to give significance to his life, a man should simply exist, and take pleasure in the beauty of simply living.
