Why am I obligated to give my money to people who haven’t earned it? Because they are in need? Because they chose not to take advantage of the wealth of opportunities available to them? Because they keep doing the things that make them poor?
There are many different answers to this question. Some cite religious doctrine (help thy neighbor), others cite ethical justifications (since I have the capacity to imagine your circumstance as my own, I cannot but therefore act…), and some cite prudential observations (fighting poverty = fighting drugs, crime, prostitution, etc. = better living environment).
Political philosophers often invoke the concept of the social contract to try and make sense of why individuals must give up certain rights (like keeping all the money they earn) in order to gain the benefits of social cooperation.
I prefer the basic notion that some burdens (like poverty) are collective, and are best confronted by collective solutions, like a redistributive taxation system. Individuals aren’t simply static units, insulated from their surroundings, enjoying their freedoms through various choices. They are social beings, living in society together, for better or worse.
Why are you obligated to give money? Why do you enjoy the privileges of citizenship while feeling exempt from its responsibilities? Why aren’t you obligated to help your fellow man?
To me, libertarianism doesn’t just supply answers to these questions which I find deeply troubling, it fails to recognize the intimately social nature of human-beingness.