change in dog toward child

Question
We have a two year old German sheared mix. He is a rescue dog and was badly treated by children and has been neutered.
We have had for seven months now and he has fitted in to the family well. We have done all the usual routines such as last through the door, not allowed onto furniture, fed after we have eaten, not allowed near table while eating,plus all the usual pecking order things. He obeys with no question and will sit, stay, lay down, wait for food until Command given with no problem.
Now the question: Recently he has gone to my daughter (14yrs) for a stroke or affection as he has done many times in the past, but now for no apparent reason he will suddenly snarl at her. (our daughter does not think she hurt him at any time but maybe a finger caught his eye? who knows) We have watched her and she is doing nothing that would harm or upset him. I will tell the dog no! and my daughter will give the command down. He obeys with no problem and stays until we release him.
Have you any idea why he has become like this?  

Answer
The first thing I would do is have the vet rule out any physical problem.

If not, there is no obvious cause or solution.  It could have to do with him maturing and wanting to move up in the pack order.  Put your move on the 14 year old first.  Surely she started her menstruation  before this.  Asking for attention can be a form of dominance.  He may just need a refresher course in your pack order.  At 14, your daughter could take him to 4-H.  In my area, clubs form soon after the first of the year. Even many urban areas have 4-H. For info look in your phone book under government listings for extension or cooperative extension offices. Ask specifically about a dog or canine club.  An hour or 2 a week in obedience drills could turn it around before it has a chance to get worse.  The instructor might see something the limitations of the net prevents me from too.