A perfectly good answer to the question "Why are you job-hunting?" is "I’m getting stagnant at my current job — I need a new challenge!"
Never, ever tell a recruiter or a prospective employer what you dislike about your job apart from the feeling that you’re no longer learning enough to keep you challenged.
If your company is reorganizing or going out of business, go ahead and say so, but if you are being mistreated at work, keep it to yourself.
A recruiter who deserves your trust will earn your trust, slowly, through their actions more than their words. Never, ever tell a recruiter or a prospective employer these ten things:
1. Never suggest or indicate that you are desperate to find a new job.
2. Never disclose the fact that you have conflict with your boss or fear you might get fired.
3. Never tell a recruiter or prospective employer that you’re not having luck with your job search or that you’ve pursued opportunities that didn’t pan out.
4. Never tell them which other companies you’re interviewing with, or where you are in the recruiting pipeline with those organizations.
5. Never tell a recruiter or employer about life circumstances that make it especially urgent for you to find a job quickly -- for instance, that your spouse or partner lost their job.
6. Never tell them that you are so interested in the job, you would take a pay cut. That’s the worst thing you can say, because it doesn’t mark you as desirable, but rather as needy — and you almost certainly will get a pay cut in that case.
7. Never tell a recruiter or employer that you need a job fast — for instance, to have a steady income in order to qualify for a mortgage. Your urgency to get hired will translate into a less appealing job offer.
8. Never tell a recruiter or employer if you plan to sue your past employer or file a claim against them. It’s none of their business, and however sympathetic they might be to your plight they will naturally wonder "If we hire this person, will they sue us one day, too?"
9. Never tell a prospective employer or a recruiter "I’m so glad this company is interested in me, because I just had a good job offer fall through."
10. Finally, never tell a recruiter or a prospective new employer that you are tired of job-hunting and just want your job search to be over.
You have to remember that your relationships with employers and recruiters are business relationships, no matter how friendly they are. You are on one side of a negotiating table, all by yourself. The employer and the recruiter sit on the other side of the table.