Bobby Singer is a man of fighting battles. His whole existence now seems to be a threaded robe of common battles. But, of all the battles he fights, the one that does the most damage is the one he doesn't even know he's engaged in.
Crowley is Bobby's destined shadow. In actuality he is a crossroad demon, one who holds more contracts than one can fathom, and he is bound to Bobby by a constant fascination and endless enthrallment. He conveys the impression of feeding off of his claimed charge's misery, happiness, anger, relief, affection, disgust and frustration. He seems to linger upon Bobby's feelings, picking through them with an untold intrigue.
If this whole façade had occurred with anyone who wasn't the burly hunter, Crowley would observe these occurring emotions until he grew bored and he will most likely kill them and move on to the next person. Yet not with Bobby.
Since Bobby's only romantic interest lay dead and he has no children, Crowley has been in the peculiar position of being a detrimental impact of the mortal's life, and in knowing that Crowley feels as if he has some brand of purpose. Some power that he has never been able to obtain before.
As such, he has been consuming the human's very energy and thusly is able to bathe in the unfathomable feeling of satisfaction whenever he pleases.
Yet Crowley was faced with constant rejection, constant forced disinterest and wariness on the human's part, and It was like his own personal apocalypse every single time.