Why does Huck fear becoming civilized in The Adventures of Huckleberry Finn?

The civilized people in Twain's Huck
Finn don't have any fun, either.  They have to do things like sit in church
and go to school.

On a more serious note, to be civilized
in the novel is to have no conscience.  Huck isn't aware of it for most of the novel,
but when he befriends Jim he is being uncivilized, according to the home he was raised
in and the society he lives in.  He is raised by people that actually own slaves, of
course. 

Huck has a conscience.  He thinks that's a
negative, but he still follows it most of the time, and rationalizes reasons why he
should. 

He never directly condemns slavery as a whole, but
he does give in to his conscience and, in the end, helps save Jim.  This is a very
uncivilized thing to do.