Subject: Why do dogs kicked their legs?

How come when you scratch a dog somewhere they kick their leg? 
Especially if it's at the site of a bug bite? It's obviously a 
sign of pleasure, but why exactly do they do it? Like, with my 
dog, she'll sometimes do it when she's chewing on a spot herself. 
And with humans for example, we have a mechanism in our brain 
that lets us know we're tickling ourselves so it doesn't tickle. 
Is this lacking in dogs? I'd appreciate if you'd answer me. 
Thanks! :)

Re: Why do dogs kicked their legs?