A tick contributes nothing to the health of the dog, but expects the dog to provide its life support.
Many people enter into marriage with similar expectations.
He's usually aware of the fact that he has certain needs. He may expect his wife to cook his food, wash his clothes, clean the house, mend his socks, and give birth to his children. He may have other unspoken expectations. Perhaps he wants her to stroke his ego, share mutual interests and make herself available to fulfill any request he makes. She may expect her husband to be the family breadwinner.