My personal belief is that resource guarding starts with fear. And resource guarding is not limited to food.
I further believe that puppies, having individual personalities, will exhibit behaviour that reflects their surroundings. A confident puppy has no use for resource guarding which is why socialization to all aspects of life is important. I have taken mine to home depot, school yards, the downtown tourist area of Niagara Falls (talk about a lot of people) and introduced them to clattering pans, vacuums, saws, drills, machinery, wheelchairs, etc. I believe it all helps with their mental development to be more secure in strange surroundings. My two youngest, that I bred, are used to people and dogs. Doesn't mean they like them, just means that they are not particularily nasty to others. However, I do believe that a bonded pair of dogs will show more aggression to other dogs trying to infiltrate their union. (being neutered may also affect this scenario as well)
That is why I tell people to socialize everywhere they possibly can. When the puppies leave here, I have no control over how people socialize their pups. The one litter I had four males, one is not good with people, dogs, strange situations, etc. The people that took them had health issues and could not socialize him properly. The other one started out reserved, but with lots of socialization and hard work on the owners part, became a well adjusted dog that is good with situations and people, dogs. The other one started getting a little to big for his britches (he was always the first one to get in trouble and a tri to boot. He had a show contract on him, but I believe a good home is so much more important than a breeding/showing contract), so, with my permission, they neutered him, socialized the heck out of him and he's now become a well adjusted dog as well. The one I kept back has always been good with people and other dogs (if Rose, his friend, is not around) He became quite bonded with Rose to the point that he is now a jerk if he can't see her. But, he is never, ever aggressive with people. I also don't put up with a lot of crap from them (just like children they need limitations) so if something becomes an issue, I'm on it right away. I don't have time to waste so I am particularily firm on certain things. Growl at me? Off the furniture and away from me until you realize that is not nice. Don't want me to touch you there? I touch all the more. So there are four different dogs, all with different personalities, with three good outcomes. The fourth could have gone either way, but due to health issues, we can't say that he couldn't have been good. (and they know and acknowledge that this is their fault).
However, with that one that was placed with the people with the health issues, a girl was also placed from a different litter and she remains very, very sweet to everything. With the same limited socialization.
That's why I say every litter, every dog, is different and it depends on how much you put into the dog to make him well adjusted. MO only.