It's a very important question and it's not the first time I've been asked it.
In our fairly free economy, people don't earn money based on how hard they work. Firstly, people earn money based on the supply of that type of labour. So low skilled employees (almost can do the job), no matter how hard the person works receive a small wage.
Do highly skilled people work harder to achieve these skills? In some cases, for example doctors or pilots (I assume). But a lot of the time they don't, for example a typical historian compared with a marketing executive.
If you take professions in the public eye for example, take a class of budding actors. 30 people all working to varying degrees. It's not that hard to imagine a film director coming along and picking the star of his next film to be the person he judges to have the most talent out of the class, as opposed to their friend who spends 20 minutes more per day practising.
That person goes on to become famous and ends up earning 100 times what the next highest earning brings in from the class. Even if that person was the most talented, and hard working, they certainly weren't 100 times harder working. It's an extreme example but something you find to a lesser degree everywhere.
A more common type of example; me and my partner. She works night shifts caring for old people in a home. She's paid minimum wage. I once put a website together in two days which made me more money everyday before I woke up in the morning than she made in a 12 hour shift. If half my money was taxed away, it wouldn't affect my quality of life much, if she had a 10% increase in taxes, she'd find it difficult to deal with.
Secondly income is based on the local economy, so someone in London earns more money than someone in... er, Nairobi. Well that's mostly just the luck of where you're born and brought up.
Thirdly... I can't think off the top of my head and this post is long enough.
So in summary, I don't think wages are fairly handed out depending on how hard you work, therefore taxing people more that earn 10 times as much for, say the same amount of time spent working (or even twice as much time working) is justified. That's why regressive tax is bad.