I wonder if there's a point at which they stop and think that they don't need one. more. thing.
I turn the mirror on myself. I have a pretty healthy income (nowhere close to high enough to be affected by a tax hike on 250k plus, though), a 1500sf house, three cars (the newest is 12 years old), and a fair share of gadgets. Spending a couple hundred dollars on something is doable without worrying about paying the mortgage.
All in all, that's awfully wealthy compared to a lot of people in this country alone. I'd still like to have more money to replace one of those cars with something newer and nicer. I wouldn't mind getting a house with even more space. A 50 inch TV instead of a 37 inch TV would be pretty sweet... I imagine it's just like that only scaled up.
I think until you get to a very select group who can basically buy anything they want without worrying about tomorrow, it's pretty easy to think 'it sure wouldn't hurt to have just a little more income'.