So far we've seen cats and dogs who have been heavily domesticated and nurtured since the very first day to trust each other. This picture however was taken in the wild of these two animals who were dumped on the street. They have been able to trust each other since the very first day, learning to protect each other and watch their guard.
The cat watches the dog's rear end while the dog looks after the cat's rear end. That way they're both protected against predators who might decide to jump them in their sleep. This has probably happened with them in the past, but they were always able to persevere against the odds. Their unique partnership has allowed them to prevail in their local habitat and have come out as the dominating species at the top of the food chain.