What I want is to take into consideration the impact my actions have on other people as I go about my life. I am not arguing that anyone needs to sacrifice beyond their means or martyr themselves (and if you owe so much in taxes that you end up homeless and starving, then something is wrong with the system and needs to be fixed, no argument there). If giving 18k to charity doesn't take away from your other responsibilities, like feeding yourself, saving for retirement, paying your mortgage, etc., and if you have any sense of obligation to that charity - say, they helped you or someone you care about when you were in a rut - then by all means, give to charity.
Also, lets be clear, giving to charity is not the same thing as paying taxes. Government's job is to protect the welfare of citizens, and can be held accountable for doing their job (via courts, elections, etc.), while charities, generally, cannot be held accountable to the same degree. Government has centralized power and authority that charities do not, and which allow government to be more effective and efficient than charities. Having a well-funded, well-functioning government is critical for my ability to earn an income, so I do feel an obligation to pay taxes. I acknowledge that it is not a perfect system - there is corruption and waste - but the solution is not to abandon the entire institution, but rather to work on making it better.
edit: could i do more and be better about how I invest disposable income? yes, definitely. does that mean I should give all my disposable income to charity? maybe, if I truly think that's what I should do. I would be a lot more comfortable giving more money to charity if I knew that there was a solid safety net to catch me if I ever became disabled or lost my job.