So this was a discussion between Arjuna and Kṛṣṇa in the Battlefield of Kurukṣetra. So the subject of discussion was that although the battle was declared, Arjuna, when he actually found that "On the other side there are my relatives," how he could slay them? Kṛṣṇa advised that "Everyone must execute his prescribed duty without consideration of any personal loss or gain." According to Vedic civilization, there are four divisions of the society. Everywhere the same divisions are there all over the world. This is very natural. Just like we can study from our own body, there is head, there is arm, there is belly, and there is leg, similarly, in the society there must be a class of men who should be considered as brain, another class of men should be there who will protect the society from danger, another class of men will be expert in producing food grains and give protection to the cows and make trade, so. And the rest class of men, namely who cannot work as brain neither can work as protector from danger, nor they can produce food grains or give protection to the cows, they are called śūdras—as you cannot avoid, to make your body complete, the brain department, the arms department, the belly department and the walking or working department.
So Arjuna belonged to the group of men who was meant for giving protection to the society. So when he was declining to fight, Arjuna, when he was declining to fight, at that time Kṛṣṇa gave him advice that "It is your duty to fight."