If your dog is exhibiting some behavior you don't want, you may have wondered, "Why is he doing it?" Does he not love you? Is he trying to dominate you? If he knows you don't like whatever it is he is doing, then why does he keep doing it? Is he not your best friend, after all? I think the answer is that he behaves the way he does simply because he has some need that the behavior helps him meet. He may not even find the behavior particularly fun to do, as is the case with most reactivity. But your dog has learned that behavior is a way to get what he wants or needs.
Think creatively about what your dog gets as a result of doing a problem behavior (whatever he's doing that you want to change). In other words, what is the functional reward for his behavior? Think of the functional reward as a "real life" consequence that reinforces the problem behavior. Has your dog learned that barking at strangers makes them move away? The fact that the person moves away creates safety in the dog's mind by putting distance between him and a stranger. That is the functional reward for his barking.
Once you know the functional reward(s) for your dog's problem behavior, the next step is to find other behaviors you can encourage your dog to do that can reasonably lead to that same reward. For example, you can reward your dog's choice to turn his head away from approaching strangers instead of barking at them. That would make looking away a replacement behavior for the problem behavior of barking. Sniffing the ground, yawning, sitting, or looking at you are also appropriate possible replacement behaviors for reactivity. Reinforce the replacement behavior(s) by using the same functional reward that your dog earned from doing the problem behavior. For example, when he looks away (a replacement behavior), happily walk your dog away from the stranger, thereby increasing the distance between dog and stranger (the functional reward).