As the title says it all, be happy always. If being sad can make the other person happy, why would you be sad?
A story to relate to the question. It might or may not depending on each person's perspective.
Let's say you joined a new school and system of learning hard and then you come to know that your teacher is strict. So you commit a mistake with the work assigned by your teacher and you do not inform since you are afraid of him. Later you create even a bigger blunder. When your Teacher notices, he shouts at you for not informing him prior. So in this case, if you cry and apologize everything will not be made better rather you apologize and reflect so that next time it does not occur again.
Being sad and crying or rather pitiful nothing actually happens. You could say that you are making your tear glands work more( he he he).
So always be happy and keep up your positive attitude.
