There is no question about being Happy or Sad - Gita 5:20

Gist: On attainment of either the desirable or undesirable, a spiritual aspirant must neither be happy, nor sad within. He is neither attached to the desirable, nor averse to the undesirable. He who realizes the true essence of the Self (Soul) and the modes of nature, knows that he is not the doer, and thus there is no question about him (the self) being happy or sad.

"Na prahrshyetpriyam paapya nodvijetpraapya chaapriyam
Sthirabuddhorasammu dho brahmavid brahmani sthitah." (Gita 5:20)

"He who neither rejoices on obtaining what is pleasant, nor grieves on receiving that which is unpleasant and who is of the firm understanding and unbewildered, such a knower of God vests in God" (Gita 5:20)

Let us focus on the first line - "Na prahrshyetpriyam paapya nodvijetpraapya chaapriyam" - To get favorable things, objects, beings, incidents, circumstances, body, senses, mind, principles, caste, creed, scriptures is called attaining that which is agreeable or desirable(priya) . To get anything contrary to that is getting what is disagreeable or undesirable (apriya).

On attainment of either the desirable or undesirable, a spiritual aspirant must neither be happy nor sad within (in the inner faculty, conscience). Having knowledge of both that which is desirable and undesirable, he is neither attached to the desirable, nor averse to the undesirable. Here the mistake is more about impressions being made on the inner faculty by being happy and sad from the desirable and undesirable. Mere knowledge of agreeable and disagreeable is not a flaw, if one remains free from joy or grief.

Knowledge of agreeable and disagreeable is in the inner faculty (antahkaran) , but the one who gets happy and sad is a doer. Though all actions are performed by the modes of nature, yet he whose mind is deluded by egoism thinks "I am the Doer," and feels happy and sad. But he who realizes the true essence of the Self (Soul) and the modes of nature, understands that it is the modes (gunas) which are acting on the modes (Gita 3:28). He knows that he is not the doer, and thus there is no question for him (the self) to be happy or sad.