The key to unlock the secret

I don't quite understand how some people see the glass half full while others see it as half empty. Without a doubt I believe that if I could figure out the answer it is the key to unlock th secret to happiness. How you see things will determine whether you have inner peace by focusing on what you do have or you have inner turmoil being stressed out over what you don't have. Figuring out the key to changing someones "point of view" sounds easy but it is almost impossible. Until they open their eyes to see things from another "point of view" they won't see clearly and they will only see the glass half full.