What reason does one have to donate their hard-earned money to charity?
The hedonic treadmill, in its application when predicting happiness given some level of income, implies that after some level of income - which for lower / medium values sees a positive, roughly linear (or starker) correlation between income and happiness - people start to experience diminishing returns in their experienced happiness as a function of income.
This is to say that, if you are anything like the average (which we have to reasonably assume) then on the hedonic treadmill theory, there exists some value of X (income) with which you will (for all larger amounts) experience considerably lower subjective happiness relative to that of what somebody with much lower income would experience, from this same amount of money.
In their paper on emotional wellbeing in which they analysed more than 450,000 responses to the Gallup-Healthways Well-Being Index, Daniel Kahneman and Angus Deaton conclude that:
Where “Emotional well-being” is “measured by questions about emotional experiences yesterday”.
For income amounts greater than this point of inflection in subjective happiness values, do we have strong grounds for keeping this extra income when we can predict that it will benefit a person of low income (/ life quality) considerably more than ourselves?
One must posit that our meagre relative gains to be made on self-interested terms for levels of income past this inflection point must morally-outweigh the potential for massive benefits by donating it as a means of helping somebody of much lower income e.g. helping rid somebody of death, pain, or disease via effective interventions, medicine, etc. for extremely low values of income.
objections on self-interested terms
The above argument alone does not hold if one cares strictly about the experienced happiness of oneself solely.
However, there is a wealth of information (such as here) that suggests charity giving is conducive even to purely self-interested goals, due to how good people cite feeling upon donating to charity.