‘And this commandment have we from him, that he who loves God love his brother also.'
So God has made it a command. God commands us to love such brothers. So it is not only logically and spiritually necessary, it is commanded. We have no choice in the matter. We are to be as one (John 17:20-23), because he has commanded it. But it should be noted of what this love consists, it consists of the keeping of the commandments (1 John 5:2-3). It is a noble, moral love that seeks wholly the good of the one loved. It is an essential part of God's whole purposes.