Their research is spiced with fancy words such as intrinsic and extrinsic motivation. Take intrinsic motivation. This type of motivation occurs when performing a physical activity is its own reward. Simple involvement is sufficient to spark interest and provide enjoyment. When you enjoy the activity, you want to do it again and again.
Extrinsic motivation is motivation too, but it is short-lived.
Things outside of you provide reasons for being physically active. Rewards, money and prizes are examples. First, see if you can identify things that make you feel as though you are not in control or against which you tend to rebel. For example, you may find that after a while, external rewards stifle your motivation because you are no longer doing physical activity for its own sake but to get the reward. The reward is controlling you.