The first time your service dog growls at you when you try to take something away from him, you feel shocked and affronted. Your first impulse may be to strike out.
"But this is my SERVICE DOG! He should be able to trusted in any situation."
Well, dogs are dogs, just like people are people. If someone tries to take something away from you that you feel is yours and is valuable to you, then you will defend your right to keep it. At the very least, you would verbally warn them that what they are doing may lead to confrontation. If someone walked up, even a family member, and just grabbed your cell phone from your hands, you would be upset, wouldn't you? You'd certainly voice your complaint. That is what your dog is doing. He is saying "This is mine, I value it highly and I don't trust you to not take it away from me." The only thing wrong with this behavior in dogs is how humans interpret the situation.
Resource Guarding is a Normal Behavior for Dogs
Resource Guarding is a normal behavior for dogs, though not a desirable in a service dog since in public, despite laws that protect your dog from being interfered with while working, the reality is that people don't think before interacting with service dogs and they don't read patches on vests etc. People of all ages may try to take things way and they may let their dogs approach your dog when he is working.
Possession is 9/10 of the Law
Among dogs, possession is nine tenths of the law. What this means is that if a high value object (food or toy) is in the personal space of a dog, (whether or not the object is in his mouth) that object is considered his. It would be rude if another dog or person came over and removed it without invitation or permission. In many cases a fight would start.
While some dogs will allow another dog to take it, it is because they know the other dog wants the object more than they do and to keep the peace, they will not fight for it. These are usually highly socialized dogs who spend time with other functionally socialized dogs.
Dogs, unless not properly socialized or they have been traumatized by another dog or their handlers, in general, are willing to do what it takes to keep the peace between themselves and other beings that have been socialized with. Fighting is risky and they may end up injured or dead, so that is why dogs have developed a complex communication system to avoid conflict. Each dog has different things that are important to him (might be food, toy or even their person) and depending on the value, he may be willing to give it up to keep the peace. But every dog has his limit. If that object happens to be the human equivalent of cell phone or iPad, then he might not want to give it up as easily and may let the other dog (or person) know by growling.
Unfortunately, not all humans have learned to speak dog as a second language and may feel it is their right to take anything away from a dog, even if it is not their dog. And even if the dog has warned them not to. Kids may run up and take an object away, or stick their hands between your service dog and a treat you are feeding him. Humans exhibit all sorts of odd behaviors in the presence of dogs. So, since you are taking your dog into public places as a service dog, you need to teach him that strangers may take valuable things away from him, and that is fine for them to do that!
Preventing Resource Guarding
As soon as your pup comes home from the breeder at 8-9 weeks, give him a few days to settle in, then start trading items with him. To take a valued toy, move in slowly but be relaxed, gently ask for the toy, take it away and at the same time present an equivalent value toy or treat in return. Praise!When the pup is reliably trading, you can ask for the toy, then delay presenting the other toy until after he has given you the one he has. That way, the second toy is a reward for giving up the first one, rather than a bribe. Mark (or click) the instant the chooses to give it up, praise and give him the other toy.
Repeat the process with higher value things like bones. Using two equal value items helps at first. On the last trade of the training session, give the pup something higher value like a treat that he can consume and you keep the toy.
If he won't give up the toy willingly, the toy, switch your approach. Let him play with the toy he has and ignore him until he has dropped the toy and walked away from it. Pick up the high value toy. Next, use a lower value toy, get some high value treats and present the toy to him. Click a soft clicker and present a food or equivalent toy reward to him as soon as he drops the toy. Repeat until he's reliably dropping the toy when he hears the clicker. Now start adding your 'drop it' cue just before you click. After several sessions of this, try just saying the drop it cue, wait for the drop it, the click and reward. Now he's started to understand the cue means to drop it.
This also starts the process of the pup learning to give an object for a retrieve (so you get two benefits for one behavior).Increase the value of the toy or bones etc. until you are able to cue drop it and your dog willingly drops it. For most dogs raw bones or a plate of human food are the highest value to them.
If your pup has come home from the breeder doing resource guarding, you need to dig into the recent history. Do the parents do this (an indication of genetics)? What type of handling did the breeder use that may have fostered this lack of trust in the pups? Were the pups handled enough in an appropriate manner? Did their kids tease the pups with food or toys? Learning he history will help you figure out the right way of approaching it and how long it might take to overcome, especially if it is an established habitat at that tender age.
Living with Other Dogs
If your dog lives with another dog, ensure that behavior around feeding times is calm. Start by feeding one in a crate or behind a baby gate, before feeding them at opposite ends of the room before moving them incrementally closer together. Both dogs need to learn the presence of another dog near his food or toys is a good thing. Treats appear when the other dog is near. When the other dog moves away, the treats stop coming.
Teaching your dogs to take turns with other dogs for doing behaviors and getting rewarded for doing so is a great way to approach it. Make sure to generalize it to many other dogs, known and unknown to your dog.
Avoid feeding your dog in public. If you are away from home, take your dog to a private location to feed him. Put him in your car, a crate or behind a locked door. This will help to keep his stress level low and prevent strange dogs and people from approaching while he eats. Teach your dog to take turns while training with other dogs. That way, when food is used for training, he will know that his turn is coming and he will be able to earn his the food ad will not get anxious about it.Choose the value of toys used in public carefully. Avoid using bones, pigs ears, stuffed Kongs etc that other dogs may find valuable unless your dog is in a location with no other dogs. If you are trying to increase duration of your dog settling in public, use hand-delivered treats instead so you can control when, how and to whom they are delivered.