Trust is gone. That's key. A dog can sense a lot of things, and if it knows you're leery around it, game over. He's alpha now, or at least he'll act like it.
Remember if/when you get another dog, that no matter what you may think their personality is going to be like, assume they are going to be an evil, maniacal little shit. The absolute worst thing pet owners do is treat their dogs like humans. For the first year of your dog's life, treat it like a wild animal that understands two things: pain and pleasure. Never underestimate a dog's ability to figure out how to get what it wants. Especially when they're teenagers (about 7 months to 1.5 years) they are going to turn 180 degrees from a pup to a crazed hound. If you've done nothing but gush over a puppy until then, good luck.
Any dog, whether it's a pit-bull or a chihuahua, needs to be treated the same way (to varying degrees, obviously). The problems happen when assumptions are made, such as "well, it's a Lab! They're friendly and fun loving all the time. We don't need to train it." ALL dogs need SOME level of training.
From Day 1, you should be handling the pup and letting others handle it. Bring it to baseball games. Have random strangers touch it and pet it. Under supervision, allow kids to handle it (gently, of course, you don't want your pup getting assaulted and having that burned into its brain). During feeding time, play with its food. During play time, allow it to have a toy, but take it away. If at any moment it growls or shows aggression, unleash the powers of Odin on it. Fire and brimstone. You need to make a lasting impression that this type of behavior Will. Not. Be. Tolerated.
Now you've set the bar. As long as you maintain these expectations, the dog should progress well. Never let your guard down, because some dogs will constantly try to dominate. And this doesn't mean one breed or one sex. They can all act that way.
I could go on and on, but you get the gist. I own a breed (German wirehairs) that has a history of being standoffish and aggressive. I also had a Welsh corgi as a family dog growing up. Of both dogs, guess which was the biter? Yes, the cute, fluffy corgi. Because we treated it like a toy and a baby, not like a wild critter. I didn't make that same mistake when I trained my wirehairs. Now my 2-year old can poke and prod either of them, and I have zero worries. Because the groundwork is done.
I'm not harping, and I certainly am not judging you for your dog. I don't know the story, and I'm not about to make assumptions. I've just seen this sort of thing happen time and time again. If there ever is another dog, you can assure it's at least set on the right path by laying ground rules at an early age.
Good luck with the Lab! I hope you're able to come to a conclusion that benefits everyone.
