When Jesus tells a hearer who has been compelled to go one mile to go two, the image is not simply that of helping someone carry a load. The underlying setting is compulsory service: a person could be pressed into carrying goods or performing transportation duties under governmental authority.
For people living under Roman rule, such compulsion could represent an unwelcome reminder that another power could make demands upon their time and labor. Jesus takes that recognizable situation and asks what happens when the person compelled to serve refuses to let coercion determine the limits of his response.