Yoga Sutras, 2.16 and 2.17

2.16:Heyam duhkham anagatam, Pain that has not yet come is avoidable.

As Patanjali says in the previous sutra, for the spiritual aspirant, finding fulfillment in the external leads to pain. Here we have the hope that pain that has not yet come (because indeed it will if we don't wake up spiritually) is avoidable. This is very similar indeed to Buddha's third noble truth. (3. The cessation of pain is attainable.)

2.17:Drashtri drisyayoh samyogah heya hetuh, The cause of the avoidable pain is the union of the Seer (Purusha) with the Seen (Prakriti).

This can be a bit confusing because isn't what we're seeking union (yoga)? Samyoga-literally true union-is explaining a misidentification of the divine spark (purusha) with creation (prakriti). Remember back to the very beginning of the yoga sutras in sutra 1.2 Patanjali tells us yoga is the quieting of the mind and in 1.3 he continues, then the Seer abides in its own nature, because 1.4: At other times the Seer appears to assume the forms of the vrittis (thoughts, fluctuations of the mind).
In my discussion of this last sutra I told the metaphor of the movie theater. If when watching a movie we lose our awareness that we are separate from the movie then we identify with the drama of the actors and suffer when they suffer and rejoice when they rejoice. When we remember our autonomy from the screen we are released from the suffering of the movie. Likewise with an unquieted mind we lose awareness of the Seer as separate from the Seen, which leads to suffering. This sutra is telling us in another way that avidya (spiritual ignorance) is the main cause of suffering (see sutra 2.4). In the next few sutras Patanjali explains why this whole drama takes place-being born as a human and having loss of memory of our true nature.