Joseph Palmer wrote:

Here is some bad dialogue:

"That's okay, I like to help people, because I'm a helpful person at
heart. Not to mention helping people makes me feel good, but I guess I
would help people anyway even if it didn't feel good, because I belive
in it. Besides, when have I ever not helped you when you needed it?"

        Oh yes, now that's bad. Sounds like something Pretty Sammy would
        say. Or maybe Mihoshi.

Which is actually a reason why having a character babbling like an idiot 
isn't necessarily bad dialogue.  Actually it's a perfectly good way to 
convey the message, "this character is an idiot".  In point of fact, 
there are no absolute rules for dialogue, except that the dialogue must 
match the person spouting it.