Yoga Sutras 2.23
Patanjali, explained by Swami Vivekananda
स्वस्वामिशक्त्योः स्वरूपोपलब्धिहेतुः संयोगः ॥
svasvāmiśaktyoḥ svarūpopalabdhihetuḥ saṃyogaḥ
Junction is the cause of the realisation of the nature of both the powers, the experienced and its Lord.
In plain words: The Self and nature show their powers only when they are joined together. This joining is what lets each be known. Our pain and pleasure come from identifying ourselves with the body.
Swami Vivekananda explains
According to this aphorism, both the powers of soul and nature become manifest when they are in conjunction. Then all manifestations are thrown out. Ignorance is the cause of this conjunction. We see every day that the cause of our pain or pleasure is always our joining ourselves with the body. If I were perfectly certain that I am not this body, I should take no notice of heat and cold, or anything of the kind. This body is a combination. It is only a fiction to say that I have one body, you another, and the sun another. The whole universe is one ocean of matter, and you are the name of a little particle, and I of another, and the sun of another. We know that this matter is continuously changing. What is forming the sun one day, the next day may form the matter of our bodies.