If there is some mathematical reason why, I would love to know. I was going off the observation that the natural logarithm approximates the kind of diminishing returns that economists generally agree applies to the utility of wealth. This means that, very roughly, the logarithm of dollars is the 'revealed preference' utility.

It was actually more of a joke about that assumption, because it suggests that a 50 dollar meal is preferred four times as much to a 3 dollar candy bar - a bit odd, but perfectly natural if you like candy bars.