Actually no, Lewis is parodying the Bible: https://www.biblehub.com/1_corinthians/13-11.htm

The Bible quote does say that, but he's poking fun at it by saying "why so serious?"

You heard it that way because that's because that's the end of 1 Corinthians 13:11:

When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things.

C.S. Lewis is playing off of a Bible quote and that became its own thing.

Because quotes get cut wherever they benefit the person repeating them. You were shown the quote from people who don't enjoy life and want others to suffer the same way.