Dogs with problem behaviors didn't come into this world like that. They developed whatever undesirable qualities because pushy, bratty, obnoxious, and disrespectful behavior was not properly addressed and interrupted when the symptoms were in their infant stages. We all have to start at 5mph long before we're flying down the highway at 70mph.
Dogs who charge and bite people at the door didn't "just" start doing that. They were allowed to bark, get way too worked up, and run back and forth to the door (as pups or newly adopted dogs) many times before the first bite. They were allowed to jump up on people at will, or push their way into your lap while you sit on the couch, scratch your leg or bark at you for attention--never learning to appreciate or respect the personal space bubble of humans. They are probably lacking structure and leadership in their life--enjoying a lot of unearned freedom and affection in the house. It is so important to understant that we can't simply look at the bite and say, "oh man, we've got a problem." We MUST see the pushy and obnoxious behavior that was going on long before that and say "THAT'S a problem!"
Leash reactivity/aggression isn't an overnight event. It's the result of many walks (prior to the reactivity problem) that were totally controlled by the dog in a bad state of mind-- going nuts when the leash is brought out, pulling you out the door, dragging you down the sidewalk, allowing the dog to zig zag back and forth marking every mailbox, sniffing every blade of grass, and getting excited by every person, car, bike, or dog they see. It doesn't start out as explosive, scary, leash behavior--but it turns into that because the pushy, non-polite, and controlling behavior of the many walks you took before was never addressed.
Next time your dog does something, think to yourself--does this seem like a problem? Is this a little pushy? Is this behavior getting worse or more dramatic each time we do something?
If you can nip a problem in the bud in its infant stages, you can really save yourself some trouble down the road when you have a serious problem in need of behavior modification. Remember, it's much easier to put out a match than a bonfire!