"Leave It" is one of the great commands. It means, "Pay attention to me and stop paying attention to whatever it is you’re paying attention to." This can apply to another dog or your dinner plate.
Now, because we tend to think of this as a "stop paying attention" situation, we often think first of correcting the dog when he focuses on the unwanted. This works - kind of. The dog will learn to break off his attention but usually only AFTER he has locked on and/or if you are close by or have him on lead.
The part we often forget about is that the dog is ENJOYING doing what he is doing, or, at the very least, highly motivated to do it - eat pizza out of the box, snarl at a strange dog, chase the squirrel. Because of this, the activity of focusing is self-rewarding.
In the scenario - dog focuses (fun), owner corrects (not fun), dog stops (not fun), owner stops correcting (not “fun”), the only fun part of this whole deal for the dog is the focusing. He’s not going to give it up easily if this is the route you take.
I read recently a post on our website, that read, "My dog is getting sneaky about eating things off the sidewalk. He knows he shouldn’t be eating it but does." But the dog is not getting sneaky; he is getting well trained. He has learned that IF you see him doing it, you will correct him. Message to the dog? Since the fun part ends when he is seen, don’t be seen.
My advice is to turn this around. See “Leave it” as a chance to teach him to focus on you. The command "Leave it" means an opportunity for your dog to earn excellent rewards. You start this by working with things he is only moderately interested in. Drop a clean sock on the ground. He looks; you say, "Leave it" back away from the sock (dog on lead please), PRAISE him to high heaven, and give a large, amazing reward (way beyond a biscuit, you’re trying to make an impression here).
Your dog is not stupid. He weighs "Sniff sock or eat chicken meat? No contest!" You want him spinning and leaping in your direction when he hears the command. When that happens, pat yourself on the back.
Once you have that level, start working it on walks. Again, begin with only moderately interesting things. He sniffs a rock (or whatever); you say "Leave it!” back away, praise! And reward as soon as he gets to you. He needs dozens of successful repetitions at this level before you move on to the next.
Don’t be afraid to withhold the reward of lackadaisical response. As soon as your dog shows you he understands a command by enthusiastic response, raise the bar! A slow response? Show him the treat! "Hey, you could have had this if you’d been paying attention!" laugh, tease him a bit, then PUT THE FOOD AWAY. Rewarding the best efforts plus withholding reward for the worst responses will build increased responsiveness in your companion.