After declaring that one is responsible for lifting up or degrading oneself (6.5), Krishna gives a path to discharge this responsibility when he says, "For him who has conquered his self, the self is his bandu (friend/relative) but for him who has not conquered his self, the self verily hostile like the enemy" (6.6). The key is conquering self. The word 'atman' meaning 'self' appears twelve times (6.5, 6.6) in an ambiguous construction allowing multiple interpretations. But, for a practitioner, the context set in the following verses would give clarity regarding the core aspect of conquering 'self'.
Krishna says, "For one who is self-controlled, the Paramatma (supersoul) is already reached, for he has attained tranquility. He is balanced in cold and heat, pleasure and pain (seetoshtna-sukhdukh), as also in honour and dishonour (maana-avamaana)(6.6)." Essentially, it means transcending the everlasting polarities.
Arjun had won many battles that gave him pleasure. But in the battle of Kurukshetra, his teachers, friends and relatives were his opponents, and thus it brought him the fear and pain of losing his own people. Krishna immediately told him (2.14) that when senses meet sense objects they create polarities of heat-cold, pleasure-pain which are transient and we should learn to tolerate them. Tolerating these transients is nothing but self control.
We get so affected by polarities of praise and criticism on a daily basis and there is no way to stop them. Hence, Krishna repeatedly emphasizes transcending them rather than identifying with them.
Our general understanding of success is that it is getting what we want. But for Krishna, it is attaining the tranquility and self-control which is aligning with Paramatma. This yardstick can be used as a measure to check our progress on the spiritual path.
Source - Daily World