How is the comma or dash used as an interrupter in this sentence? John wept for joy when he heard that his lovable, cuddly sheep dog was safe.

The comma here is used to intensify the love/adoration of John for this dog. The fact that 2 adjectives were used to describe the dog, requested the comma to be added, therefore the comma and the adjectives form a "relation", symbiosis in order to intensify john's feelings for the sheep dog.