Maybe I just pay too much attention, but I feel like this happens about every other time I'm in the F or D1 cabin.
I think it mostly stems from a misconception of the generally flying public about the cost of flying upfront. I know I have had friends who just can't understand how I, as a 29-year old, can fly D1 or F....well, F is mostly just because I get upgraded or because I need the flexibility, and international D1 is because it actually makes a difference in productivity when I arrive and being self-employed the incremental cost isn't that bad because of the tax write-off, but to make that clear...I usually just give up and tell them I booked with miles.