cat bites himself when we scratch his back!

Question
Hi Becky, I hope this is something you can answer for me!  We have a tomcat who is around six or seven years old, in good health.  A couple of weeks ago, we noticed that when we pet his back, well more like scratch it I guess, he likes it when we start at the nape of his neck and just scratch a little all the way down to almost his tail.  Anyway, once we get about halfway down his back, he suddenly starts to lick or bite/nibble at himself on his sides, like around the top of his arms.  He stops as soon as we stop.  The weird thing is if you are scratching down the left side of his back, he will nibble himself on his left side, and if we scratch the right side, he nibbles his right.  If you scratch really slowly down his back, you can totally tell when the "spot" is coming up, because he will turn his head to that side, and start to lick his lips, and then as soon as you hit that midway point on his back he starts licking and or/nibbling.  It's so weird!  It doesn't seem like he's in pain or that it bothers him horribly, because as soon as we stop scratching, he stops nibbling, and then goes back to his regular self, cuddling or whatever.  He's an incredibly good natured cat and has never really had any "problems", so this just makes me a bit nervous.  I just wanted to know if this is maybe kind of like what some dogs do, like when you scratch a certain spot they start to scratch themselves or their leg twitches or whatever, or if it could be a deeper problem.  I couldn't find anything like this on the internet, just stuff about fleas, but he is an inside cat and never ever scratches at himself any other time, so fleas aren't an issue.  Thanks for any help you can give me!!

Answer
Hi Stephanie!

I'm not sure what causes him to do this, but it sounds to me likd he enjoys it and it's some kind of contentment reaction.   I really don't know, but my guess is ,  he enjoys it.   Who knows?  Cats have their own unique personality!  

God Bless,
Becky