it depends what profession you are in.
I know people who worked the same job for 20 years, get laid off, and are totally unmarketable.
Perfect interview scenario:
Interviewer: By the looks of your resume it seems you change jobs often?
Me: Yes it does. I'm always on the look out for new challenges and new opportunities. Plus, it highlights my wide range of knowledge and experience that i bring to the table. I know i can add value to your firm over and beyond what other candidates can because of my diverse work history.
Interviewer: You are hired.