In this story Krishna instructs Arjuna to fight, even though it is against his brother. He also explains Karma Yoga. In the passage, Karma Yoga must be action for duty, without the hope of eventual reward of any kind. Krishna wants Hindus to worth without attachment, and to be even-tempered in success and failure. Also this may not sound extremely difficult, it is not easy to continue doing something without any rewards back. Personally, I would find it very hard to go to school and work hard, if it did not lead to graduation and the hope of going to a good college. But in Hinduism, they say that the reward at the end of the road will be realizing that you are already one with Brahman and achieving Moksha.