Sometimes we pay attention to what we don't want. e.g. I don't want to be overwieight, poor, unemployed, unhappy, etc. And even when we say it's what we want (I want to stop eating junk food, I want to quit smoking), in fact, it's still about what we don't want.
When part of you is focused on what you don't want, some of your energy is now used 'against' it. This leaves you less energy to create and achieve what you do want.
Becoming clear and focused on what you DO want directs 100% of your energy and attention toward it. And by appreciating the positive aspects already there, you increase that energy even more. So if you want to be happy, notice where you are already happy. If you want to be healthy, notice how you are already healthy. If you want more money, appreciate the money you already have.
The things you focus on - and the things you appreciate - will naturally grow and expand in your life.