Notions of heat and cold, of pain and pleasure, are born, O son of Kunti, only of the contact of the senses with their objects. They have a beginning and an end. They are impermanent in their nature. Bear them patiently, O descendant of Bharata. (Bhagavad Gita 2.14)

What is pleasure for you may be pain for somebody else. What is pain for you may be pleasure for somebody else. Also, what you found pleasurable sometime in the past, you don’t enjoy as much now. And what you enjoy now might be something you hated in the past. Pleasure and pain, likes and dislikes, these are just notions of the mind. They appear and disappear. They are impermanent. Even heat and cold are just notions of the mind. For example, in the place where I live in, a temperature of 30 °C is pretty acceptable, even cool, while the same temperature would be very hot to a person used to living near the Arctic. It’s the same temperature, but what makes it hot or cold is just the notions of our mind and our habitual responses. The number that denotes the measure of temperature might be real, but ‘heat’ is not real, it’s just a response of the mind. Even the number and system we use to measure temperature is made up by us, human beings. In absolute terms, these things like heat and cold, pleasure and pain, have no meaning whatsoever.

That person who is the same in pain and pleasure, whom these cannot disturb, alone is able, O great amongst men [Arjuna], to attain to immortality. (2.15)

The person who understands that pain and pleasure are just notions of the mind, and dis-identifies himself from these notions completely, is not disturbed by these notions anymore (though the notions themselves may still exist). When the person rises above these notions, he will find the real Self which is immortal. Note that not much of effort is required to free oneself from the notions of pleasure and pain. It’s just the understanding and the spontaneous dis-identification with the concepts of the mind.

See also, An attempt to understand the Self with the help of the analogy of life-cycle of a motorcycle.

Narendra — All is one, isn’t it?

Sri Ramakrishna — No, beyond one and two.

(source)

When the mind perceives duality then there is both duality and its counterpart, which is unity. When the mind drops the perception of duality there is neither duality nor unity. When one is firmly established in the oneness of the infinite consciousness, whether he is quiet or actively engaged in work, then he is considered to be at peace with himself.

— Yoga Vāsiṣṭha (tr. by Swami Venkatesananda, p.66)

The Vedas deal mainly with the subject of the three modes of material nature. O Arjuna, become transcendental to these three modes. Be free from all dualities and from all anxieties for gain and safety, and be established in the self.

— Bhagavad Gita 2.45