Reasons why I'm currently looking for a new job:
1) I'm looking to change locations, half way across the USA
2) I've been in my current position for nearly five years, and am ready for a change:
2a) In my chosen trade (being a unix sysadmin) you often get tremendous latitude in selecting tech, tools, and tasks, but cross pollinating with the technology stack at other companies gives you answers to questions you didn't even know you had and makes you a better employee for it.
2b) I'm a disbeliever in the "you've got to get out by the time you're forty" age discrimination stuff. I have, however, seen a lot of people, both young and old, that have spent long enough in one industry that they were not hireable for a position in mine because they'd spent so long in a single frame of mind that they fell flat on their face after several attempts to coax them through design/architecture questions.
Think about it this way: if you're already an employee of the company and someone applies who doesn't really want to work there, and HR lets them through, would you want to work with them? I wouldn't.
Sure, if someone actively doesn't want the position they applied for they should have told the recruiter "thanks but no thanks", but, frankly, if I find a job I'm interested in, the interview is mutual: while you're determining if I can reboot routers with style and grace, I'm determining if you're a sweat shop, if my potential teammates are idiots, and if my future boss is a jerk. In my opinion it's the employer's job to sell me on why I want to work in that industry, rather than the other way around.