It's what they all say, right? "I just found a better opportunity."
It's certainly a logical reason for leaving. But it's also usually a symptom of a larger issue.
The opposite of "better" is "worse," and the opposite of "opportunity" is "setback."
So it begs the question, what is so worse about your workplace? And what makes it a setback?
In other words, what can you do to help build a work environment so good that people never want to leave?