Sorrow for yourself is gained by hurting others. In the same manner, victory in every war will result in another war. So also, any happiness that you can give to others will result in happiness for yourself in the end. Man must realise that he cannot get anything without sharing it with humanity around him. So, you must believe that happiness of the people around you will lead to your own happiness in due course.
Death is implied in life and sorrow is implied in pleasure.
So also, night is implied in day. We regard sorrow and happiness, night and day as different entities. No, that is not right. On a proper enquiry, we will find that in fact they are the same. Iron, in its native state is black and hard but when you heat it, it becomes red and soft.