why is my dog peeing on my bed

OK, here’s the story. My girlfriend has a 1-1/2 year old Cavalier. He is neutered and fully housebroken, and is not known to have accidents in the house.
However in the past week, he’s peed on two beds in the house. I’m sure it’s not a lack of being let outside frequently enough. Is there another explanation for why a fully housebroken dog would make a mess on his owners’ beds?

There are a couple possible explanations. First would be to take him to a vet to rule out possible physical explanations, such as a urinary tract infection, kidney problems, etc.

Second possibility is psychological. Does he have any reason to feel recently threatened/ ignored, either due to the presence of another dog, or perhaps due to your presence? Were you spending more time over there, and his mistress spending less time with him? Was he newly forbidden from sleeping on the beds?

There are a lot of questions I’d need to know the answers to before I could really advise you:

Is he peeing just on your side of the bed? I’ve seen this happen in cases where the dog disliked the new occupant. Keep the bedroom door closed to keep him from doing it again, and make an effort to become friends.

Play fetch, rub his belly, give him yummy treats, and tell him what a good dog he is. In short, win him over.

Is he actually squatting in one single spot, or is it more like a long dribble as he walks along the surface, leaving a trail behind him? If it’s the second option, it could be simple excitement, best delt with by keeping him off the furniture until he controls himself a bit better. Keep the bedroom doors closed.
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Is the dog doing this right in front of you?* If he is, watch his ears. If they’re slicked back against his head and he’s sort of hunched and cowering looking, it may be something called “submissive urination.” Do a search on this term on the web, and you’ll find lots of pages telling you how to deal with it.