Maintaining motivation over time takes effort from both the employer and the employee.
Some motivating factors are intrinsically temporary, such as the excitement of a new job or the challenge of a deadline. The additional motivation that these things bring will be lost, at least until the next new project or deadline.
Other motivating factors are more long-lasting, like having a positive working environment, a supportive and helpful boss, or a fair and constructive bonus scheme, and generally feeling valued at the company.
Employers also have to consider the demotivation of boredom, particularly in repetitive or unchanging roles. Giving employees new duties or the chance of promotion can help to counteract this problem.