Most people don't believe this principle. It's something you have to try in order to see it work ~ a bit like learning to swim: you have to immerse yourself in water to realise its ability to support your body.
If you've ever hoped to change someone, you'll have experienced the frustration and disappointment of failure. If you've ever been on the receiving end, you'll know how irritating, if not downright infuriating, it can be. We resist change at the best of times, but we defend our behaviour to the end because we are seldom able to distinguish between it and our psyche, so inevitably feel under attack when our behaviour is questioned.
Consider a typical situation: To Mary, Jim has become somewhat distant, hiding his emotions and communicating less and less about the things Mary considers important. She has tried to get Jim to open up, encouraging him to talk about his worries, but this has only led to arguments that end with Jim storming off.
Mary confides in her friends. They all agree it would be better for Jim to share his burden; they consider Mary's desire to change Jim's behaviour is entirely justified and encourage her to have another go "for his own good". This of course leads to a huge row and Jim withdraws once again, muttering about how no one understands him.
What Mary fails to realise is that she contributed to Jim's withdrawal. Back in the days when they did communicate, she became quite critical of Jim's expressed views ~ or at least that's the way Jim interpreted what she said. Actually, Mary was simply much better at grasping the issues and could not understand why Jim so often failed to see what was going on right under his nose.
Jim has never analysed his behaviour, but has developed a subconscious strategy for damage limitation: "I get criticised for expressing an opinion, therefore I will escape criticism by keeping quiet."
Their current situation confuses both Jim and Mary. She cannot understand why Jim has become so withdrawn; he considers it unreasonable to be attacked for keeping his head down, when that was surely what she wanted?
Mary will never change Jim's behaviour by imposition. The more she tries, the more he will withdraw. It's a lose-lose situation, but there is a winning alternative.
If Mary will look inside herself and silence her own critic, she will make it safe for Jim to express himself once more. Only she need instigate change for both parties to change their behaviour, and only she can do this. Jim will, quite inexplicably, open up to her once Mary changes her own behaviour.
This example suggests one party to be at fault, but that's not so. These behaviours are hidden from their exhibitors. We all behave in ways of which we are unaware. It is our willingness to listen to those who love us enough to confront us with objectivity and our willingness to adopt more appropriate behaviour that will make our world a much more satisfying place to have fun.