Personally I regularly browse Armys I currently down own on Games Workshops website, and for most of them I can see myself going down that line of "I'd like to paint those models" or "I could just have a small 500 point force...." that inevitably leads to more shelf room being taken up and hiding bank statements from my better half for the next few months.
I sort of feel the same way about Necrons and Tomb Kings in Fantasy too, which leads me to think my main driver for buying an army is the 'Narrative' as Games Workshop obsessively brands it these days. If I start a new Army, I generally buy Black Library novels about them also so that I can learn more about the fluff whilst I build my new force.
What about you guys? What Army could you never play and why? Is it aestetics, mechanics, or something else?
Let me know!