It's polite to split the cost of things.
John and his friend take a taxi together and his friend asks John to pay half the fare.
John wants to reply to his friend about the taxi fare.
John says he'll pay half the cost of the taxi ride and his friend pays the rest.
John says his friend should pay the whole fare because it was their idea to take a taxi, so they reluctantly do so.
John's friend thinks that each person paying half the cost is fair and thinks John is being reasonable.
John's friend thinks John isn't fair for not paying half the cost of the taxi that he used.