This is one of the most important lessons you have to learn. An example will help to demonstrate the point. Imagine that you want to write a book. Before you choose this goal, you rarely heard anything about this tipic. However, after you get this idea it's in your mind constantly. You notice an ad on television for writers, your ears perk up when your friends talk about a book and you become interested immediately.
You see, when you think of something you begin to see the world in a different light. Before I become involved in motivation, I rarely heard the word. Now that I have dedicated much time on the topic, I see, hear, read and think of the word hundreds of times a day. It's all because what I chose to think about.
Remember, it works the other way as well. If you choose to think negatively, you will see examples all around you that reinforce your negative and damaging thoughts. You are that you think, so think the right things.