What does it mean to be 'surprised by joy'?

"Joy" does not mean "everything is going well." 

CS Lewis, upon his conversion, had this to say about the mystery that is "joy."

“In fact I meant by ‘things going well’ just that security – or illusion of security – which you also regard as unhealthy. Real joy seems to me almost as unlike security or prosperity as it is unlike agony.

“It jumps under one’s ribs and tickles down one’s back and makes one forget meals and keeps one (delightedly) sleepless o’ nights. It shocks one awake when the other puts one to sleep. My private table is one second of joy is worth 12 hours of Pleasure."

https://www.theguardian.com/books/2014/dec/09/unseen-cs-lewis-letter-defines-joy-surprised-by-joy