The next two sutras deal with the principles of suffering:
II.15 Pariṇāma tāpa saṁskāra duḥkaiḥ guṇavṛtti virodhāt ca duḥkham eva sarvaṁ vivekinaḥ
The wise man knows that owing to fluctuations, the qualities of nature, and subliminal impressions, even pleasant experiences are tinged with sorrow, and he keeps aloof from them.
II.16 Heyaṁ duḥkham anāgatam
The pains which are yet to come can be and are to be avoided.
Saṁskāras, subliminal impressions, are the latent imprints deposited in the mind of every past experience of pleasure and suffering. When these ripen as karma and fructify, one again experiences pleasure and suffering. The desire for pleasure and aversion to pain trigger these latent saṁskāras, which fructify and become memories of past pleasures or pains. These memories generate fresh craving or aversion, the desire to re-create past pleasurable experiences (or avoid known miseries). This is a form of mental torment
One might give the example of a person who wakes up with a terrible hangover after a night of alcohol excess, swears he or she will never drink again, but, come the next weekend, is back at the bar, destined to awake with another hangover the next morning. The saṁskāras of so-called pleasure produced from a night on the town are imprinted in one’s citta mind. When these reactivate, if one’s desire to enjoy in this way is sufficiently strong, it overrides discernment, or the memory of the negative consequences, and one again feels the urge. Thus the cycle of attempting to find happiness but, instead, ultimately being subject to suffering, is perpetuated. The compulsion to experience happiness is so strong that one typically resigns oneself to the inevitable inconveniences that accompany it, under the rationalization, “That’s just life.”
Bryant, Edwin F.. The Yoga Sutras of Patañjali: A New Edition, Translation, and Commentary (pp. 347-348). Farrar, Straus and Giroux. Kindle Edition.