The Bhagavad Gita - II - 66

नास्ति बुद्धिरयुक्तस्य नाचायुक्तस्य भावना।
नचाभावयत: शंति: अशान्तस्य कुतः सुखं।।

 Where is joy when the intellect is unconnected

For the one who is not rooted in the truth, the Buddhi or intellect is non-functional. When the Buddhi does not function, there is no possibility of having a firm picture of the divine self, whether in a form or without it. The Buddhi is the one that gives a definite picture. In the place of the Buddhi, the mind rules. The mind as you know is never steady. It is always wavering with its innumerable thoughts. It means, such a person knows no peace. When there is no peace of mind, where can one expect to experience bliss?
The equation goes like this - an unconnected intellect cannot have a divine glimpse of the truth. It is replaced by the wavering mind. The mind is unsteady and hence knows no peace. When there is no peace, there is no experience of bliss too!
 - SS

Bhagavad Gita - II - 65

Prasade Sarva Duhkhanam Hanirasyopa Jayate|
Prasanna Chetaso hyashu Buddhih Paryavatishthate||

In that state of divine blessedness, all sorrow is destroyed. In its place, there is a a cheerful and enthusiastic state of mind that surrounds the intellect and heart. Enthusiasm is the greatest religion. It is an offshoot of intense Shraddha and faith. Without enthusiasm, the intellect loses its fuel that makes it stand straight. When all the impurities of the mind that keep the intellect on shaky ground is gone, there is nothing else left for the mind to be steady and this steady mind is the state of a Sthitha Prajna.

- Swahilya Shambhavi.