Sometimes I'm impatient with others whose standards are not as high as mine. Interviewees show their true colors. First of all, I want to learn your business from the ground up. Long term, I hope to be in a position of greater responsibility that is mutually beneficial to the company and to myself. Any negative about a prior employer is an absolute turndown immediately.
Bad apples offer sour grapes, to mix a metaphor.
The employer is fishing for information here. He or she is giving you the opportunity to prove that you offer the most important prerequisite for this specific job. The recruiter may be worried that you are vocationally immature or that you don't really know what it is like to fill a job. If you worked as, say, a lifeguard, he or she may be concerned about your lack of drive and ambition. Actually, my experience as a guard was a valuable leadership training ground.