Apparently I had missed this question that a reader had posted on the blog, but I'm going to try to answer it now.
I am new at your blog, but I have questions regarding my just adopted
dog. He was very sweet when we first got him, but now he seems to be
aggressive. He is nice when I am at home with him (I work at home), but
once my husband comes home from work, he is very aggressive towards me,
growling and bearing teeth. He is not neutered yet as he was found to
be heartworm positive when we got him and undergoing treatment for that.
Why do you think he is aggressive towards me when my husband is home
and not when my husband is gone?
This is actually a very interesting situation, and it may be somewhat difficult to answer. First, I have a few questions of my own:
1) How old is your dog?
2) What breed is your dog?
3) What do you and your husband do for a living?
Without having met you or your dog, if I had to guess, I'd probably say your dog is exhibiting some guarding behavior. Namely, your dog is guarding your husband. Generally, when I see this, it's when a dog has bonded more with one person over another. The dog essentially "claims" this person as his and feels it's his job to protect this person and his property. You'll often see other behaviors, generally mistaken for affection, as well. These may include climbing into the person's lap, sleeping next to the person, or insistence on being petted. In most cases, you'll see this type of behavior displayed towards a woman, while the man in the house is shown aggression. Of course, though, there are no rules.
So, what do we do about it?
Well, we first need to teach your dog some manners. I generally say it's not about what you DON'T want him to do, but rather about what you WANT him to do. In other words, it's easier to tell him to sit than it is to tell him to stop. So, try to teach him some basic obedience. All he really needs to know is one really great command (down/stay or something similar). If he can do this through any situation, you're ready to move on to the next stage.
Next, create some distance between your husband and your dog. Instead of allowing your dog to climb up in your husbands lap or climb between you and your husband when you're relaxing, tell your dog to down/stay on the other side of the room. Your husband could also be the one to tell him this. Teach him that his biggest rewards will come when he's calm and away from either you or your husband. Remember, it's not about teaching him to leave your husband alone. Rather, it's about teaching him that good things will happen when he's not trying to guard your husband.
After that, we need to teach him that all good things come from you and he has to work for them. I call this plan The No Free Lunch Program. Essentially, have him do a command for everything. Make him sit before you pet him. Make him sit before you play with him. Make him walk in a heel when you take him out. Make him sit/stay before you take him outside. Make him sit/stay before you feed him. Have him down/stay while you're eating. Of course, you don't have to use the commands I just stated, but you do have to have him work.
Lastly, you can amplify the No Free Lunch Program a bit. Feed him out of your hand rather than out of a bowl and have him do a command for each new handful. Don't allow him on furniture unless specifically invited or don't allow him at all.
By teaching him to follow you more and that you're the provider of all things good you'll create a much stronger bond with your dog, and you'll encounter fewer incidents.
One last word of caution: Take note of whether the aggression is worse at certain times of day. If you're dealing with some sort of neurological disorder or physical ailment, things may be worse in the evening when your dog is tired and less tolerant of outside stimuli.
I hope that you and your dog can find a peaceful cohabitation.