Why not? Simple – the company that’s making you the counter could care less about you the moment you resign. You’re tainted goods that they’ve now had to pay a premium to hang on to. If you think they’re going to just forgive and forget, you’re delusional. Paranoia runs deep. Every time you step out to make a private call, or take a sick-day, they’re going to assume you’re interviewing. If the firm runs into some trouble, and needs to make lay-offs, you’ll be on the list – guaranteed. You’re expensive, tagged as disloyal, and they won’t feel guilty about letting you go since you clearly wanted to at some point anyways.
Think about the reasons you were looking in the first place. If it was money, do you want to stay somewhere that you have to use blackmail (and that’s what using an offer to get a counter is, kids) to get a raise? If it’s environment, do you think the company is going to suddenly shift itself to accommodate you? Easier for them to make some hollow promises, and then quietly call an agency to start looking for your replacement against the moment you wise up that they were just talking nice. You made a decision, and while a counter-offer can make your head spin, keep a written list of why you are leaving and pull it out whenever you get tempted.
Quick, hard fact: when I was on the agency side, I periodically had a candidate who took a counter. 90% (and I’m not inflating that, I tracked the percents ‘cuz I’m obsessive) of these candidates called me within 6 months because “things didn’t change” “they don’t trust me” “I just got laid off”, etc al. I refused to work with any of them – to my mind they weren’t trustworthy, hadn’t listened to my expert advice when I told them that this was exactly what would happen, and were frankly dumb about their careers.
Write a resignation letter that makes it clear you are leaving, and that it’s final. Be polite but firm. If you want a free template, e-mail me at firstname.lastname@example.org, or leave a comment, and I’ll get you one.