Ousel and Bertie you're right of course. :) I don't mean to sound draconian or cold, because I'm really not at all. And of course I talk to DS about his behavior and why it is wrong, and tell him things like we do not hit people in this house, I'm the mummy and it's my job to love you and keep you safe and bring you up to be a good person and you need to do as you're told for that to happen, etc.
But he is little and does not always understand, and for lots of things the reason then becomes "because I'm your mother and that's the way I want it." For instance, we allow a very little bit of television on weekdays. DS often wants to watch more. I tell him no. I explain that we are going to do other fun things. He refuses and starts to tantrums. I've explained to him that lots of TV is not god for your mind, the same way as lots of chocolate is not good for your teeth. He understands, to some degree, but he doesn't care because he's three. And yes, because I'm bigger I can enforce it, and if he pushes the issue then he goes to his room, because I think it's annoying to have a three year old screaming at me because I won't allow one more episode of Diego.
Also I have worked with lots of kids who were allowed to do what they wanted all the time. You'd think they'd be happy, but most of them were quite miserable. It's a lot to put on a small child, to be responsible for themselves. They don't know how.
That being said, I do know one set of parents who don't "discipline" as such and their daughter is very well behaved and happy. It works for them, and that's fabulous. It just wouldn't for me. :)