Why did Jesus call the Old Testament command to love God and each other a new commandment?

The writings of 1 John 2:7-9 shed insight onto this new but old commandment.

7 Dear friends, I am not writing you a new command but an old one, which you have had since the beginning. This old command is the message you have heard. 8 Yet I am writing you a new command; its truth is seen in him and in you, because the darkness is passing and the true light is already shining. 9 Anyone who claims to be in the light but hates a brother or sister is still in the darkness. [1 John 2:7-9]

Likewise in the Sermon on the Mount, Jesus presented the old testament laws in a new way. The differentiation is between following the letter of the law as opposed to knowing the meaning and intent behind the law.