Chapter 2, Verse 15
Part of 2: Sānkhya Yog — सांख्ययोगयं हि न व्यथयन्त्येते पुरुषं पुरुषर्षभ। समदुःखसुखं धीरं सोऽमृतत्वाय कल्पते॥
yaṁ hi na vyathayantyete puruṣhaṁ puruṣharṣhabha sama-duḥkha-sukhaṁ dhīraṁ so ’mṛitatvāya kalpate
That firm man, whom surely these afflictions do not, O chief among men, to whom pleasure and pain are the same, is fit for attaining immortality.
-- Dehadhyasa or identification of the Self with the body is the cause of pleasure and pain. The more you are able to identify yourself with the immortal, allpervading Self, the less will you be affected by the pairs of opposites (Dvandvas, pleasure and pain, etc.)Titiksha or the power of endurance develops the willpower. Calm endurance in pleasure and pain, and heat and cold is one of the alifications of an aspirant on the path of Jnana Yoga. It is one of the Shatsampat or sixfold virtues. It is a condition of right knowledge. Titiksha by itself cannot give you Moksha or liberation, but still, when coupled with discrimination and dispassion, it becomes a means to the attainment of Immortality or knowledge of the Self. (Cf. XVII.53)