How we cite our quotes:
She could not believe that the young boy laughing in her face was Ellis Wyatt. The tense, scornful face she remembered, now had the purity, the eagerness, the joyous benevolence of a child in the kind of world for which he had been intended. (18.104.22.168)
Ellis has been transformed by Atlantis, which is closely linked to themes of youth and childhood. The idea of regaining the happiness of youth appears throughout the book, as well as the idea that happiness shouldn't die with youth.
She heard the rising, accelerating sound of the wheels – and some theme of music, heard to the rhythm of the wheels, kept tugging at her mind, growing louder – it burst suddenly within the cab, but she knew that it was only in her mind: the Fifth Concerto by Richard Halley – she thought: did he write it for this? had he known a feeling such as this? (22.214.171.124)
A lot of important symbols are tied together here: motion, the railroad, and music. All of these things embody happiness for Dagny since they express her values.
What he knew, what he had discovered tonight, was that his recaptured love of existence had not been given back to him by the return of his desire for her – but that the desire had returned after he had regained his world, the love, the values and the sense of his world – and that the desire was not an answer to her body, but a celebration of himself and of his will to live. (126.96.36.199)
Hank unites themes of love, desire, sex, and happiness here. Love and sex can be a way to express happiness and to "celebrate" life.