I think he was saying that he enjoys the intellectual stimulation of solving the very hardest, no-one-else-can-solve-them cases, and being on the cutting edge of his uber-sub-specialized field.
He's afraid that FM won't provide intellectual stimulation for me, and I'll be bored.
Again, very black & white thinking.
You'd be surprised at how many cases FP's solve after patients have bounced from specialists to specialists, $80k of medical costs later... putting their families closer to bankruptcy.
You don't have to be uber-sub-specialized to have a job that's intellectually stimulating. Having an intellectually stimulating job depends on YOU, not the specialty you chose. If you're the type that accepts dogma, jumps to conclusion, never question authority... then no matter what job you take, it'll likely be intellectually dormant.
If you have the guts to ask "why", or "what if", or "how about", or take a medical problem X and add to the end "in the primary care setting", then your job starts to become more interesting. And what appeared on first blush "simple" becomes more complex.
Have some beers with techies and business-y types. They do this all the time. They talk about doing things better, faster, cheaper. It's how things advance and innovate. That's what we need to do.
If you want to be intellectually stimulated, keep an open mind; and dare yourself to challenge, seek certain truths, and find a faculty who'll support you and mentor you through it. And when you find your answers, talk about it, teach it, publish it.
Before you know it, you're doing research! And some medical student will think that YOU're on the cutting edge! And patients, students, and residents will be banging down the door to be near you. Creating your own destiny, and inspiring others to do the same.
I find it very hard to believe that a field so broad, affecting so many people, every day, in every manner... can be so boring (according to your attending). Medicine and the health care system is so jacked up that there's opportunities everywhere to make things better, faster, and cheaper.
Maybe that was the case back in the day when your attending was going through training. But Barack is president now. And like he said, "time has come to set aside childish things... Starting today, we must pick ourselves up, dust ourselves off, and begin again the work of remaking America."