Why Mankind Was Created as One

The Ohr HaChaim asks why Hashem first created Adom and Chava as a single being and only later separated them, while the animals were created as male and female from the start. He answers that the Torah is marking the difference between human and animal relationships. Animals have many mates, and we never speak of one ewe as the wife of a certain ram. Only among people does a man and a woman form a lasting bond and build a family. To teach this, Hashem formed them from one body and then divided them, so that they would seek to rejoin as one flesh.