Once upon a time, there was a snake who at the end of a path wanted to cross a river. The river was raging, and it looked hopeless.
Along came a horse down the path, looking to cross that river. The snake said “HEY! Could you help a brother out?”
The horse was accommodating. He carried that snake across that river safely to the other side, where he dismounted and promptly bit the horse.
The horse was dumbfounded. “Why did you bite me?” he asked.
The snake replied “I’m a snake, stupid.”
Indeed, as one might trust those who are recipients of favor, there is a fairly consistent expectation that simply feeding an alligator wont protect you when he is hungry again. In this case of carrying the snake, its in his nature to bite.