We know the answer doesn't have to do with force because we have not identified a positive or negative axis.
Depending on what the system we are evaluating is, the sign of the work can change.
If work is done on the system, it is negative, but if work is done by the system, it is positive.
If the system we are evaluating is the leash, the work is being done by the leash, and therefore, the work is positive.
If the system we are evaluating is the dog, the work is being done on the dog, and therefore, the work is negative.
