Notice how Matthew 7:12 begins: so whatever you wish. That little word so is doing a great deal of work. It reaches back not just over the verses right before it but over everything Jesus has said since chapter five. Blessed are the poor in spirit. You are the salt of the earth. Love your enemies. Do not be anxious. Judge not. Now Jesus gathers all of it into one sentence.
Back in Matthew 5:17 he said he came not to abolish the law and the prophets but to fulfill them. Here in 7:12 he says this rule is the law and the prophets. Commentators note that those two verses form a bracket around the entire body of the sermon. Everything between 5:17 and 7:12 sits inside that frame. And this verse lands right before Jesus calls for a decision: the narrow gate, the two trees, the two builders. It is the final complete thought before he asks which path you will take.
When you read Matthew 7:12, you are not reading a stray saying. You are standing at the summit of the Sermon on the Mount, where Jesus draws every thread together and turns to look you in the eye.