As humans we make choices every time we interact with our companion animals. A standard catch phrase of training is "every interaction is training". That is, don't expect your dog to sit at a door as it's being opened if you don't teach him or her to sit at a door every time it opens.
Positive trainers believe that working with companion animals who are choosing to stay in the game is the best way for real, and meaningful, learning to take place. It builds understanding, duration, the ability to generalize and all kinds of good things when the animal can make the choice to participate freely.
This neat little video reinforces and explains how this principle of choice works when you are
playing training flighted wild birds. If they can do it surely we all can?