ok cool, basically it just follows the idea of diminishing returns to utility, whereby, for the most part, the more you earn the less you value each additional dollar you get; Ie/ it derives to you a dimishing positive return. Not everyone is like this, but if you take away selfish motives and irrational decions such as power grabs etc then its a fair description of why tax should be weighted relative to income. Say you earn 20k, a 2k rise in your income will be valued much higher by you than the person on 80k. My folks are relativly high income earners, so my view isnt distorted by my circumstance, even though I am conservative and an economic liberal I do see the advantage of taxing higher income earners higher, even though it is, in some ways, penalizing people for working harder.
Theres other things as well I can scrounge up probably, to do with social cost and benefit, where by taxing those who have a diminishing returns to utility of income higher will lead to a better social outcome, and taxing those who have an inceasing returns to utility less. The theories have flaws and dont always account for human behaviour, but I think they sort of describe by stance on the issue decently.