Whatever you are focusing on, is where your energy goes. Whatever you allow in your space, you eventually become.
When you focus on something negative, you will eventually be consumed by it. When you feed your brain a constant diet of negative thoughts such as self-criticism, you will become someone with low self esteem. If you focus your anger or jealousy towards someone, it will eventually become the only thing you can think of. Why do that to yourself?
Focus on the good, the positive, your dreams or goals and you will eventually become the person you wanted to be.