If you have to force yourself to do something, you are less willing or less able to exert self-control when the next challenge presents itself.

i.e., unless you're given a strong incentive to do so, people will often feel demotivated to continue with tasks, even if it's towards a goal that they want to achieve.

This is one reason why you're far more likely to snack on ice cream in the evenings, after a long day, than in the morning. Your ability to make a healthy decision (self-control) has been impeded.