I believe that a person's ability to choose to be happy can be compared to predicting the weather. If someone asks you, "will it be sunny tomorrow?", You might say sure, only to wake up the next day to heavy rains and dark skies. In certain situations happiness, like a sunny day, can be a person's choice. An example is a bad hair day or a missed opportunity. Other times, like the rain and thunder, life makes it hard to choose happiness. The death of a loved one or the abandonment of a parent are just a couple of the situations I myself have encountered. I believe you can tell your brain to be happy , but your heart has a mind of its own.
