Whether you look at your teacup half full or half empty, whether you react to the grumpy cashier at the check-out line with compassion (she is having a bad day, who knows what happened at home), or send her angry signal right back to her with an irritated reaction - the choice is always your's. Your reactions come from your beliefs. Every thought in your mind is a cause to an effect down the road. I mentioned in a recent related post that researchers found that depressed people are depressed because they have negative thoughts; they do not have negative thoughts because they are depressed. That difference is crucial! Why? Because you can change your thoughts - once you are aware of them.
Once we accept the responsibility of our thoughts, and that they create our reality, we are no longer at their mercy. Telling your mind what to think or not to think is one of the things that meditation teaches. If you do not rein your mind in it gallops away with whatever comes along - and that can create a reality that is out of your control and not to your liking. Or you can dig deeper into yourself and become aware of what you are actually thinking.
Think about it.