Yoga Sutras 2.17
Patanjali, explained by Swami Vivekananda
द्रष्टृदृश्ययोः संयोगो हेयहेतुः ॥
draṣṭṛdṛśyayoḥ saṃyogo heyahetuḥ
The cause of that which is to be avoided is the junction of the seer and the seen.
In plain words: The suffering we should avoid is caused by the joining of the seer and the seen. The seer is our true Self, called Purusha. The seen is everything in nature, from the mind down to the body and the world. The pure Self only seems to feel pleasure and pain because it is mixed up with the mind.
Swami Vivekananda explains
Who is the seer? The Self of man, the Purusha. What is the seen? The whole of nature beginning with the mind, down to gross matter. All pleasure and pain arise from the junction between this Purusha and the mind. The Purusha, you must remember, according to this philosophy, is pure; when joined to nature, it appears to feel pleasure or pain by reflection.