That has to be one of the grossest scenes to imagine. It is bad enough for the dog to get sick, and its first inclination after vomiting must be to get away. But, why on earth would the dog ever (and I mean ever) return to the vomit? What possible appeal or benefit could it ever offer?
Peter quotes this proverb (yes, “As a dog returns to his own vomit” is from Proverbs 26:11) in 2 Peter 2:22 to picture a Christian, who has “escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ,” but then he is “again entangled in them and overcome” (2:20). Why on earth would a Christian ever return to the pollutions of this world once he had escaped? What possible appeal or benefit could the world ever offer? Exactly the same appeal and benefit that the dog would receive from his own vomit!
The imagery is gross to think about, which is why, I believe, God chose such a strong language to describe how seriously dreadful it is for a Christian to turn from God.