What is this one problem that leads to all the others?

It is force – especially when its use (and threatened use, which has the same effect) is legalized.

If force institutionalized were taken away, there would of course still be disagreements – as well as annoyances. But that is all there would be. Or at least, mostly. Violence – its use and its threatened use – will of course always be with us as there will always be people willing to resort to the use of force to get what they want. But they would be outliers – and occasional rather than ubiquitous threats.

How often have you been mugged?

Have you been able to go even one day without being forced to pay what are styled "taxes" – i.e., legalized mugging? You are forced to pay the "mugger" every single time you pay for anything. And when you get paid for the work you do, too.

Because such mugging it is legal – and so ubiquitous. The mugger in the alley is a kind of freelancer; he enjoys no such advantage and suffers the disadvantage of it being legal to attempt to avoid his depredations and to defend oneself against them.

If you don't like someone, but he is legally powerless to force you to interact with him, then you are free to avoid him. And because you are able to avoid him, conflict is avoided.

How do you feel about people you're forced to deal with – because they have the power to force you to deal with them? For example, the proliferation of government employees whose employment centers on your having to not only deal with them politely but also pay for the privilege?