Final answer:
Cognitive evaluation theory is the motivation theory that best explains the decrease in Sam's overall level of motivation when he started being paid for previously enjoyable work.
Step-by-step explanation:
The motivation theory that best explains the decrease in Sam's overall level of motivation is cognitive evaluation theory. According to this theory, when extrinsic motivators, such as payment, are introduced for tasks that were previously enjoyed for their own sake, intrinsic motivation can decrease.
This can be attributed to the concept known as the overjustification effect, where the introduction of external rewards undermines an individual's intrinsic motivation for the task. In Sam's case, being paid for work that he previously enjoyed doing may have shifted his focus from the inherent pleasure of the task to the external reward, resulting in a decrease in his overall motivation.
To illustrate, let's say Sam used to enjoy painting as a hobby. Tonya, his employer, starts paying him for his paintings. Now, Sam's motivation to paint might shift from the pure joy of the creative process to the desire to earn money. As a result, he may feel less intrinsically motivated to paint and his overall motivation may decrease.