Our external Universe is merely a reflection of our internal one.

Have you ever noticed how people who feel sad tend to perceive the world as sad, and people who feel happy tend to perceive the world as happy? The facts of the outside world are nor happy or sad, they are neutral. It is our perceptions that give it the flavour that it then has to us, and qualify it as happy or sad. That means that by changing ourselves, we actually change the world, because it is a reflection of what's inside of us. And that explains why nothing changes until we change, and everything changes once we've changed.