My pet is a public menace, what did I do wrong?

In my current attempt at nethack I'm playing a dwarven samurai with a dog pet, so most dwarves I've encountered are peaceful. However, for some reason, my dog has been going beserk on every friendly dwarf we meet and I'd like to know why.

Since this doesn't really incur penalties on me, not to mention he was getting experience, I didn't mind it while going down the gnomish mines, but now it's becoming a problem. I've just reached Minetown and imagine my surprise when my pet decides to attack the owner of the first shop I visit. Though this still incurs no penalties, I'm afraid he's going to get zapped to death, so I have to lock him out of every shop I visit.

Now I wonder, did I do something wrong that is causing this?
I didn't catch any message like "Rex is confused with hunger." or something (plus, I almost starved because he was pouncing on every single corpse we made).
I've heard you can train your pet to steal by feeding him when he drops you stuff, is it somehow possible that I have accidentally trained him to be aggressive?

Solution 1:

As far as I know, there's no way to train or otherwise cause your pet to be more aggressive. In general, pets don't much care if a monster is friendly to you or not - they'll go ahead and attack anyway, as long as the monster is close to their level.

It's quite possible that your dog has reached max level by this point in the game, especially if he's been getting a lot of kills. However, dogs don't normally attack shopkeepers, even at max level.

I can think of a couple possible explanations for your situation:

• You're wearing a ring of conflict. (Unlikely, since you'd be getting attacked too.)
• Your pet was at maximum level and ate a wraith corpse. This would put him within a level of the shopkeepers and liable to attack.

Some pertinent info from the wiki article on pets:

They say discretion is the better part of valor. Pets don't always possess such discretion, and often find themselves overpowered by opponents they take on. Although a pet will not attack a monster which could kill it in one round, this calculation takes no account of the monster's weapon or passive attack. Be especially careful about bringing dogs or cats into the Gnomish Mines, or horses into shops, especially in the deeper levels. Horses seem especially vulnerable to mimics, and have also been known to attack shopkeepers, as they reach a higher level than dogs or cats. (A pet will not attack a monster which is two or more levels higher than itself. A fully grown large cat or dog is level 9, a fully grown warhorse is level 10, and a shopkeeper is level 11.)

also

Carnivorous and omnivorous pets will eat wraith corpses and gain one level for each, capped at 15 over base level. (There is no cap for gains from engulfing live wraiths.)