This is an incredible power we have: whatever we focus on, we amplify.
This phenomenon works for us both within ourselves, and interpersonally. And it works both negatively and positively.
When I am down, I see the world through a negative lens, and consequently find more to be down about. When I am actively looking for the positive, I see more positive. So when I did the 100 Days of Happiness project, taking a picture of something that made me happy 100 days in a row, I started to build awareness around those things, and so I started seeing them more. This is what Gratitude research is all about; when we log three things we are grateful for every day, we are honing our focus, building awareness, and so we start seeing more of what we focus on.
Interpersonally, when I get focused on what I judge as negative in you, I am showing you a frustrated, angry, withheld part of me, all hard edges and cold eyes, which of course makes you tense, self-conscious, and resentful (for example), guaranteed to bring out the worst in you. When I focus on the positive in you, and especially when I actively reinforce it, you feel the warmth and good energy, which helps you relax, be yourself, and respond in kind. So simple, yet so powerful.
To fully understand this dynamic is to recognize a profound power that we all have.