Arousal Theory
As an expansion to drive-reduction theory, psychologists Stanley Schachter and Jerome E. Singer developed the arousal theory of motivation, which tacks on the idea that humans are also motivated by various levels of arousal. This theory investigates the influence of neurotransmitter dopamine on human motivation. In this context, the term arousal refers to the psychological state of being more alert and stimulated, and dopamine is a chemical compound in the brain that is associated with transmitting a message of pleasure.
The focus of this theory is on the level of sensitivity to rewards or goal-achievement that the human mental state facilitates. Fulfilling a goal or accomplishing something always has a level of biological arousal, or neurotransmission of dopamine in the brain, and this motivates individuals to make certain decisions or take specific actions in order to achieve this effect. You tend to engage in activities that are physiologically arousing or make you feel good. This theory is essentially oriented as pleasure-seeking as a motivation for human behavior.
Follow this theory when you need to motivate yourself when you are feeling down. Figure out what makes you feel good. Whether it is training for the goal of completing a half marathon or taking time to finish a book, focus on the pleasure that comes from accomplishing goals to elevate your mood.