Are you an extortionist? Asked another way: have you ever accepted a counteroffer?
Imagine that you have just come into a hefty sum of money. Maybe you've won the lottery or had a rich uncle pass away. A local newspaper writes a short article detailing your sudden enrichment and quotes you on what you'll do with the money. The article even includes a photo of you beaming at the camera or holding a comically large check.
A nefarious fellow reads of your windfall in that paper. You've known this guy for years and have developed a relationship with him and yet he decides that he would like to have some of that money. He shows up on your porch one evening. You open the door only to be greeted by a pistol in your face. He politely asks for some of your money.
How do you feel now? Frightened? Betrayed? Thrown into survival mode? Willing to say or do anything to have this immediate situation go your way?
If you have ever accepted a counteroffer from an employer, you are the nefarious fellow on the porch. Some aspect of your current job has induced you to interview with other potential employers, accept an offer, and give your notice. All those feelings you had in the first hypothetical are now being felt by your employer. They've know you for years. You have friends in the office and now you want to leave?
They're afraid of what it's going to take to replace you. Of course they feel betrayed; they didn't even know you were looking! Survival mode kicks in. They can't have you leave. You do things in the office that nobody else can and they really don't want to suffer the time and expense of recruiting for and vetting your replacement. So they say and do whatever they can to make the immediate situation work out in their favor... they make a counteroffer.
You accept it because, heck, you've been there for years, know your way around, and have friends in the office. You think that everything will be just like before except that you'll make more money. Except that doesn't happen. That betrayal lingers. The company might start searching for your replacement (only cheaper), or you might start remembering why, aside from the money, you started to look for another job in the first place. It falls apart, and within 18 months, you're no longer with that company one way or the other.
Depending upon whose studies you believe, this is precisely what happens to 65% - 90% of those who accept a counteroffer. Within 18 months of accepting a counteroffer, they're gone. If you decide to leave, leave. If you like your job but need to make more money, talk with your boss about your performance and a raise. Please do not be a nefarious fellow on a porch using the threat of leaving instead of a pistol. Odds are that it won't work out in the long run.