Why do dogs do this?!?

Like most dogs, Gigi will bark her head off when she sees someone walking by our home or up the walkway to our door. And she'll growl bettween barks. Then, the minute I open the door to whomever it is, she will get all happy and jump on them like they're her best friend. Yeah, some watch dog!

Why do they do this?

I think they know that you let the person in, and they didn't come in themselves. Also, I think the barks are meant to do two things, one, intimidate the 'intruder', and two, alert you to the fact someone is there. If you don't react negatively, I think they pick up on that.

Some dogs are just super friendly, though. My dog loves people way too much to be a guard dog. He may look tough, but he just want to have fun play with everyone he sees.

Dogs are territorial, so they like to let people know that this is their domain, and of course, the only way they can do this is by barking. They also pick up on the signals you send, so if you're relaxed and welcoming, then so are they. I expect if you were less accommodating - maybe if the caller was a sales person or a missionary - the dog may continue to growl or bark. However, they also bark when they're pleased to see someone, so it's a question of listening for the tone.

Dogs are territorial as Sandra said. From what I've read, they know the difference between people you greet and let into your house and people you don't. This is why they bark at the mail carrier and delivery people. They come up on your porch, but you never let them in the house.

It sounds like you are talking about strangers though, right? If I opened the door to a stranger, my dog would not stop barking. Even my previous dog who had better manners would bark. Though, she would have taken my cue as to when it was okay to stop. My Jack Russell? Not so much. Hopefully, your dog would know the difference if a threat was posed. Most dogs do without any special training.

In some ways this is good. If you get a religious or salesperson, they are usually deterred by the barking dog. I had a group of Jehovah Witnesses come half way up our walkway, stop to hear the dog, then turn around. Good doggie