You have a better offer.
Professionals who are good at their jobs get noticed. Companies want to employ the best in their field, after all, and where better to find the best in the business than within a competitor's upper ranks? If you do good work and are skilled at networking within your industry, there's a good chance you'll be noticed by competitors.
A better offer may come as a surprise. If you're good at your current job, odds are you're happy there. But another company that wants you bad enough may be willing to offer whatever it takes -- more money, more flexibility or better benefits -- to convince you to join its team.
If you're approached with a better offer, don't be afraid to ask tough questions about the company's business and its work environment. You're the one holding the power, after all: You can simply reject the offer and stay in your current position if you so choose.
A good outside offer may put you in position to ask for more from your current employer, as well. It's common practice -- and considered courteous in most industries -- to give your present employer a chance to counter-offer and keep you on staff. Negotiate right, and you could end up gaining a raise, promotion or other benefits without having to change jobs. If your current employer won't negotiate, however, you should take that as a sign that you may be better off in a new job.