Agitations of the Mind
You know both happiness and unhappiness. Only that one who is apart from the two can realize both. He who becomes one with either may remain eternally happy or he may remain eternally unhappy. But he who remains even-minded in happiness and in unhappiness, he can know both, that happiness and that sorrow. Happiness is separate and unhappiness is separate. And he that remains apart from both these can know them both. If we blend with them, then he will be unable to know both happiness and unhappiness. Rather we will know only one, the state in which we settle.
Second point, while experiencing happiness you are the same one as while experiencing unhappiness. And it is only because you are the same one, that you are able to experience them both separately. The one who experiences both happiness and sorrow separately, is apart from happiness and unhappiness. Now when will you have separate experience of happiness and unhappiness? When you no longer remain established in Nature (Prakriti), and become established in Self "Samdukhasukha swasthah" (Gita 14:24).
Nature is constantly changing. If you become established in it, then you will constantly undergo agitations and changes. But that agitation will never be in you (self). Even in a state of ignorance, these disturbances are not in you and even in state of perfect knowledge, these disturbances are not in you. In your Self (swarup) there were never any agitations, can never ever be any agitations. If there were agitations and disturbances then they could never be wiped out. These agitations, disturbances and changes are happening in Nature (Prakriti). To experience your Self as being apart from Nature (Prakriti) is the essence of Knowledge. It is to attain Salvation.
In reality you are apart from Nature (Prakriti). To understand this point, you please have mercy on me. Please pay a little attention. You are not the one to come and go.
It is the subject of the sense organs to give happiness and sorrow through favorable and unfavorable situations. They come and go and they are not permanent; therefore you endure them.
Maatresparshaarstu kaunteya, shitoshnasukhadukhadaa
Aagamapaayinonityaa staanstitikshasva bharat
(Gita 2:14)