Holding a motor that is lifting a weight will indeed increase the load on your arm due to the principles of physics, specifically Newton's third law of motion. When the motor exerts force to lift the weight, that force is transmitted through your arm, making you feel the additional weight. The example of the man with the donkey illustrates the concept of shared load and burden. Analyzing the situation with free body diagrams can clarify the forces at play. Ultimately, the additional weight felt in your arm is a direct result of the motor's lifting action.