Some translations soften the end of Matthew 7:12 to say this sums up the law and the prophets. But Jesus says it is the law and the prophets, their true meaning and essence. Summarizing the law was a popular game among the teachers of his day. They loved to debate which single command was greatest, which one to keep if you could keep only one. Jesus is doing something different. He does not say this is the greatest command among many. He says this one principle actually is the law and the prophets.
He is drawing the principle up from Leviticus 19:18, you shall love your neighbor as yourself. That command sits underneath every ethical demand in the Old Testament, and the apostles never forgot it. Paul wrote that love does no wrong to a neighbor, therefore love is the fulfilling of the law, in Romans 13:10. He wrote in Galatians 5:14 that the whole law is fulfilled in one word, you shall love your neighbor as yourself. James called it the royal law in James 2:8. Later Jesus himself will say that all the law and the prophets hang on loving God and loving your neighbor.
This means you cannot keep God’s commands toward your neighbor while ignoring this sentence, because this sentence is what those commands are about. To try to obey them without love is to miss the entire point.