To Huck, civilization represents repression and lack of autonomy. It also represent the rule of women. To Huck, it is cluttered with rules and rituals that don't make much sense. As he puts it, about living with the Widow Douglas:
The widow rung a bell for supper, and you had to come to time. When you got to the table you couldn't go right to eating, but you had to wait for the widow to tuck down her head and grumble a little over the victuals, though there warn't really anything the matter with them.
On the river, in contrast, Huck experiences a sense of freedom. It's not that he lives a disordered life, but that he is in charge and can follow a routine that makes sense to him. As he describes it:
Here is the way we put in the time. It was a monstrous big river down there—sometimes a mile and a half wide; we run nights, and laid up and hid daytimes; soon as night was most gone we stopped navigating and tied up . . . Then we set out the lines. Next we slid into the river and had a swim, so as to freshen up and cool off; then we set down on the sandy bottom where the water was about knee deep, and watched the daylight come.
On the river, Huck no longer has to live by other people's rules and expectations. He is his own boss.
But more profoundly, civilization represents a moral quandary for Huck. He has innocently imbibed its "morality," such as that it is a sin to help a slave escape to freedom. On the river, living outside of the constraints of civilization, he is able to develop a wider moral vision. The morality of his civilization condemns him and makes him feel guilty: he worries he will roast in hell for helping Jim. While we can laugh with and at his chafing under table manners and such conventions, we feel for the way the larger evils that are domesticated in the civilization he knows cramp the very possibility of the kind of expansive, humane relationship he is able to develop with Jim in the liminal space of the river. It is this deep sense of relationship we tend to respond to as readers, and we can understand the value of stepping outside of the boundaries of the known (i.e., the civilized) and into a world that allows our souls to expand.