Thoughts have energy

I see it like this:
You think positively, therefore, you notice the positive things happening in your life more vividly. You grow to appreciate the positive things, no matter how small.
When you think negatively, you notice the negative things, and even sometimes make a situation more negative because that is how you're looking at your life and your goals. You focus so much on the negative things happening in your life, that the positive things just wave sadly as you fly by.