Your life has changed in a major way.
Suppose you get married to someone who works in another state, or your spouse gets a tremendously good job offer that requires you to move. Perhaps you're about to have a child, or an aging parent is moving in with you. You want to spend more time with your family, and your current job situation may not allow for that kind of shift in your priorities.
Smart employees alert their bosses to these life changes early on, keeping them in the loop as their needs, priorities and availability shifts. Beyond being a courtesy to your employer, this practice can greatly improve your odds of adapting your work situation to your new life situation. Perhaps your employer will have enough time to adapt your job requirements or help you find a new job within the company that better meets your life needs. If that isn't possible, your employer may become a powerful reference as you search for a new job. In some cases, your employer might even be willing to help you find a new job, cultivating a long-term relationship in case you're able to return in the future.
If, however, you encounter resistance to the change from your employer, what does that say about your current job? It may be easier to endure the stress of a job change if you learn that your employer doesn't respect your need for a healthy work-life balance or expects you to put your job before personal and family commitments.