As I said before, verse 12 is an appropriate concluding statement to this entire section on not judging others, not being a hypocrite, and knowing whether to confront someone about their sin. Not only because it nicely sums up Jesus' earlier exhortations but because this is the wisdom God will reveal to you when you ask, seek and knock on the heavenly door in prayer. When you seek out wisdom about what to say to a brother who might be sinning, how to humble your own heart so as to avoid hypocrisy, or whether to refrain from saying anything at all, chances are the rule God will reveal to you can be summed up as: "However you want people to treat you, so treat them."
What's so brilliant about the way Jesus puts this teaching is you don't have to be a particularly "sensitive" type or a genius at psychoanalysis to follow this rule. We're all good at knowing how we prefer others to treat us. We're pros at remembering all the times someone else made us feel irked or slighted or angered or hurt. From there all we have to do is take the extra step of making sure we don't do the same to someone else. That extra step is the hard part, of course, but at least there is no mystery about how to love the other person. "Treat others the way you want to be treated" is closely parallel to Jesus' teaching "love your neighbor as yourself," except it is worded more concretely and brings into immediate focus what your love for the other person ought to look like.
It's amazing the epiphany you go through once you decide to put this teaching into practice. All the doors of your relationships open up, especially in evangelism. But for me the biggest inital hurdle was taking the Golden Rule (as it's called) seriously in the first place. It just seemed so indulgent. Of course I want to be treated with total sensitivity and with the utmost respect for my feelings and be cut plenty of slack for my blunders. It couldn't be right that my obviously selfish standards be turned around and used as the standards by which I am obligated to treat others.
Yet that's exactly what the New Testament teaches. "Be devoted to one another in brotherly love, give preference to one another in honor . . . bless those who curse you, bless and curse not, rejoice with those who rejoice, and weep with those who weep . . . never pay back evil for evil to anyone . . . never take your own revenge . . ." (Romans 12:10, 14, 15, 17, 19). I could also quote 1 Corinthians 13 but I'm sure you get the picture. It's all about overflowing, over-abundant, gushing-like-a-fountain grace and love and forgiveness toward others.
I used to think treating others with such generosity was "indulgent," but maybe that's because my view of God was too miserly. In this teaching Jesus seems to be implying that the natural craving we all have for how we wish to be treated reflects a true desire for the love of God himself. And if we respond to that God-given craving in others, they cannot help but recognize the divine origins of our love.