Is that true anymore? Everyone I know is always on the job market, even folks who haven't jumped employers in 10+ years are regularly in idle conversation with the job market.
But, the honest answer to your boss's question: what bothers me is trading my time for money. My comp doubled. I can now afford to stop working in 3-5 years instead of 10+ years. I think this attitude has become much more common in the last 5-10 years.