I don't like losing good employees, but when one tells me he's thinking of going somewhere, whether it's a competitor, school. different industry, whatever, I don't take it all personally. Sooner or later, I do the same thing. And that's the reason my people will tell me when they're considering something else.
P.S. My rule of girlfriends and bosses: You learn more about them when you leave than at any other time.