Have you ever had a dog who only gets mad when you give attention/affection to a certain pet?
My sister has 4 dogs. Buchi is the aggressive one. When we pet the other dog named Whitey, he will go ballistic and bark endlessly... even though we're far far away. However, when we pet the others- namely Chooey and Rin- he will totally ignore it as if they aren't there.
I wouldn't say mad but definitely jealous! With me it's Kotton, the oldest at 4yrs old, she was an only for 3 years and I swear she plans ways to ship her sisters out. If I give any of the others attention Kotton is right there trying to nose them out of the way. With my girlfriend it is Kash, the youngest and definitely a momma's girl. If my girlfriend tries to give attention or kiss any of us (yes me included LOL) Kash tries to use her body to muscle the intruder out...sometimes it's cute because she does it so slick. You know the yawn and put your arm around someone routine? Yea well Kash does the stretch thing too. Koko is just oblivious as usual and just waits her turn
Luckily I have never had that problem unless you count when my boys are both trying to sit in my lap, which ends up with them both there. I do not play favorites or pay more attention to one of them more than the other. In my opinion this is what leads to a jealous dog. Of course I have never had a dog that required more attention because of social or health issues. Situations like this could promote a feeling of jealousy or another dog fighting for additional attention. Pets, not just dogs all need to feel they are equally important and have the same amount of attention of their owners.