It seems that people are able to empathise somewhat with GAD, but not with NPD.
My immediate response to this is that it's hard to empathise with NPD simply because people with NPD don't often know or believe they have it.
GAD is something that most people can imagine. Everybody has felt anxiety (or most people have perhaps) at some level, and most people are able to project up and imagine how debilitating that would be if it was a constant sensation.
It's really hard to imagine thinking 'I am amazing, and the world is here to revolve around me.' Not in confident sense, but in a desperate, needing constant reassurance that you're amazing.
It's only been in the past few years that I've been looking into NPD more. I do actually feel sympathy for people who have it. To have that overwhelming, constant need for love/approval/positive reinforcement to the extent that you'll lie and manipulate and attack everyone around you just so that you can feel elevated yourself.
However, that sympathy soon turns to banging a head against a brick wall, because one of the major factors is that they can't recognise it in themselves, so any help or guidance you try to give is rejected. It makes it hard.