To answer this question with the thoughtful consideration a liberal might, lets evaluate this question in three areas. First, within an historical context, do higher income taxes stand the test of time? Then a philosophical view for why a higher income tax is not only legitimate, but also morally justified by the government. Lastly, through the eyes of the current economic landscape: Do current conditions today favor higher income taxes, or do they favor lower income taxes. There is a rich history on the American Income Tax, but for the purposes of this question, it is imperative that we look at our country's history with the income tax from it's inception to today.
The income tax first began with the 16th amendment, "The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration." Now that we know when the income tax began (1913), we can look at it in terms of high and low. High meaning anything above 60 and low meaning anything below 40. The moments in time on the line graph that correlate with higher income taxes are considered to many liberals in the country as "the good old days," while the moments in time when income taxes are low are almost always hit by recessions or depressions.
When looking at the philosophical liberal view on higher income taxes, a person only has to listen to the words of progressive Republican President Teddy Roosevelt: "The really big fortune, the swollen fortune, by the mere fact of its size, acquires qualities which differentiate it in kind as well as in degree from what is possessed by men of relatively small means. Therefore, I believe in a graduated income tax on big fortunes..." In other more modern terms, it is in the Governments and the people's best interest to have higher income taxes, because it levels out the money in a more equal way, and more evenly dispersed between all the people.
And lastly, in terms of how our country currently looks, would a liberal contend that higher income taxes are what this country really needs. Studies currently show money to be at its most disproportionate levels since the Great Depression. In fact, average CEO pay has outgrown average worker pay by a ratio of about 300 to 1. No wonder 20 percent of the country controls 80 percent of the wealth. Many liberals feel that If things keep going this way, the economy will never level out, and people will continue to suffer.
So, these are three reasons for why a liberal might want to have higher income taxes. Because history shows that more people benefit with higher income taxes, because it is morally just for "swollen fortunes" to spread the wealth, and because current economic conditions demand that we do. Higher income taxes is really the nations way of paying down it's debt. It's not fun, but sometimes it has to be done.