I have found that it's always best to give a positive reason for leaving your job for another one, then stick to that when rejecting counteroffers. I left my last job because my new job is in the field where I'm guiding my career, and it's only 4 minutes from my house instead of 45. When I received the counteroffer, I could honestly say I enjoyed working for them, but the 2 things I was going after at my new job were things they could not offer. They understood, and said they'd hire me back if I was ever interested.