There are any number of passages in the Upanishads where you will find this atmosphere of zest and joy and vigour. The outlook of the Upanishads is characterized by joy and cheer, by what William James called ‘healthy-mindedness’. God’s name itself is joy, in the Upanishads. What a beautiful exposition of divine nature is found in the Taittiriya Upanishad (II. 7. 1):

रसो वै स: । रसं ह्येवायं लब्ध्वाऽनन्दी भवति । को ह्येवान्यात्क: प्राण्यात् । यदेष आकाश आनन्दो न स्यात् ।

(Raso vai sah; rasam hyevayam labdhvanandi bhavati; Ko hyevanyat kah praṇyat; yadesa akasa anando na syat)

'He is, verily, bliss; man, verily, is blissful by getting this bliss. Who would have lived, who would have breathed, if this infinite expanse of bliss were not there?'

This Upanishad says that the nature of God is bliss itself, sense life, are but particles of that infinite bliss of God. It begins to give what may be term­ed a calculus of happiness. ‘Let us take a youth, a good-mannered youth, well-educated; full of hope, firm in mind, and strong in body; let him have dominion over the full wealth of this earth; that is the unit of human bliss.’

Having at last defined its unit of human happiness, the Upanishad proceeds to estimate all other forms of happiness in terms of multiples of a hundred of this one; in this ascending series comes the happiness of angels and gods and all orders of higher beings, reaching up to Prajapati, the Cosmic Person. But equal to the happiness of each one of these, from the youth up to Prajapati, says the Upanishad, is the happiness of a man who has realised Brahman, God, the Self of all, and has ceased to be a slave of his senses and his sense-bound mind. Spiritual realisation con­fers immeasurable happiness, as it connects one with Brahman, God, which is the ocean of all bliss, of which all others are but particles. And every youth is heir to this attainment, in virtue of which he ranks higher than even the angels or gods. This is the highest excellence of man, say the Upanishads.

The moment I realise myself as one with all, a new zest comes to me, more intense, more pervasive, and more pure. To make others happy is my happiness, to serve others and help them to achieve their life’s fulfilment is my fulfilment. This is the essential teaching of the Upanishads and the Bhagavad Gita.

Reference: The Message of the Upanishads by Swami Ranganathananda (p.83-86)