Answer
The primary purpose of life is to attain happiness. If one can attain divine happiness, that is even better! When a person is eating and an extremely hungry and frail person appears before him, and he gives away a portion or all of his food to that person, a divine happiness is experienced. Anyone can try this experiment and see for themselves.
Some people attain great happiness for themselves but often at the cost of causing suffering to others. That is the lowest form of happiness. Through such conduct, both this world and the next are ruined.
According to the Gita, if any work is performed solely as a sacred duty, for the fulfillment of the Lord's command, and for the Lord's pleasure, then a Karma Yogi always derives happiness from performing that work. He has no personal happiness or unhappiness of his own. Whatever is his duty and whatever is righteous — he always finds happiness in doing it. If someone has to do something for another person, for the other's happiness, and then feels unhappy thinking, "My own happiness has been lost," then that person has not yet entered the path of Karma Yoga.
Nevertheless, this can be a good beginning — doing some actions for others' happiness and some for one's own.
When a person lives in a family, the family members often have to sacrifice their own happiness and work for the happiness of others — whether those "others" are one's mother, father, husband, wife, son, or daughter. Only then can family life be happy.
The body of Rishi Dadhichi had become hard as a thunderbolt through his tapas (austerities). During the war between the Devas and Asuras, Indra was advised that if a vajra (thunderbolt weapon) could be fashioned from Dadhichi's bones, it would help win the battle. Dadhichi sacrificed his very life and his bones to make that weapon. Rama joyfully accepted exile to the forest for the happiness of his stepmother Kaikeyi. Shri Rama found happiness even in that exile by contemplating the joys it would bring — the beauty of nature, meeting sages, and engaging in discussions of wisdom. These were all divine characters. Even if a human being cannot immediately become that divine, he should still try, by sacrificing his own happiness for another's, to observe whether he truly feels happy or not.