Why do Henry and Lenina both say "everyone is happy now"?

Lenina and Henry agree that conditioning has made everyone happy.

"Of course they don't. How can they? They don't know what it's like being anything else. We'd mind, of course. But then we've been differently conditioned. Besides, we start with a different heredity."

"I'm glad I'm not an Epsilon," said Lenina, with conviction.

"And if you were an Epsilon," said Henry, "your conditioning would have made you no less thankful that you weren't a Beta or an Alpha." He put his forward propeller into gear and headed the machine towards London. Behind them, in the west, the crimson and orange were almost faded; a dark bank of cloud had crept into the zenith. As they flew over the crematorium, the plane shot upwards on the column of hot air rising from the chimneys, only to fall as suddenly when it passed into the descending chill beyond.

"What a marvellous switchback!" Lenina laughed delightedly.

But Henry's tone was almost, for a moment, melancholy. "Do you know what that switchback was?" he said. "It was some human being finally and definitely disappearing. Going up in a squirt of hot gas. It would be curious to know who it was–a man or a woman, an Alpha or an Epsilon. …" He sighed. Then, in a resolutely cheerful voice, "Anyhow," he concluded, "there's one thing we can be certain of; whoever he may have been, he was happy when he was alive. Everybody's happy now."

"Yes, everybody's happy now," echoed Lenina. They had heard the words repeated a hundred and fifty times every night for twelve years.