You can gain a broader base of knowledge.
Think about the learning curve you experienced when you started your current job. There was likely a period of fast-paced adaptation, followed by a longer period of learning the finer details of your work. Ideally, you ended this phase by moving into a level of mastery of your daily tasks and became an expert in your part of the organization's operation.
But are you capable of more? And could you advance your personal base of career-specific knowledge by repeating this process in a new position? The answer could be "yes."
Some researchers suggest that the typical worker masters his or her specific job over the course of three years. After that point, the pace of industry-focused learning and skill mastery slows. It stands to reason, they argue, that changing jobs after that three-year span ends resets and recharges the process, giving you the opportunity to grow and learn at a rapid pace for another three years in the new job [source: National Executive Resources Inc.].
While the three-year term isn't a hard-and-fast rule, it does provide a loose guide and a reference point to evaluate what you're learning at work. Would changing to a new position within your industry force you to learn new skills? Could those skills complement those you already have, making you a more well-rounded and capable professional? If so, this could be a good reason to start looking for a new job.