Unfairness creeps in when taxation is not used simply to raise money to provide services and benefits for all members of society, but is used as an incentive to make people, or some people, behave in a certain way.
To give a simple example, a tax allowance or rebate may be given to new start up businesses, because the government wants more people to start up new businesses.
But the tax allowance may be seen by existing businesses to be unfair because they are having to pay higher taxes than the new business.
The perception of unfairness leads to perverse incentives and efforts to avoid paying tax.
So perhaps a starting point could be to stop using taxes to change people's behaviour, and concentrate on the primary function of raising money.