Because we are humans and some of us won't consent so it must be forced. There are people who feel that rape, murder, and arson are all things they should be allowed to do. They don't consent to the laws. That's why they are in prison. It's a necessary thing, assuming you want a functional society.

You mean by moving somewhere else?

The point of laws in a democracy is that they're (theoretically, without corruption) consented to by a maximum amount of people. If you don't have rulers, then whatever your group of 10 people consent to as an unwritten rule will be broken by me, much to my gain.

So basically social norms and conventions.

All the parties who agreed to the contract are responsible for enforcing it.