When your sims is friendly as trait and has a grumpy face thinking of getting friends. xD
When your sims improve a shower for a friend and he has a grumpy face suddently at you when you tell him. Then he tells you hello and after he is grumpy again.
The hot-headed sims are often grumpy. :(
other logic from pets.
When you are a one star celebrity because a friend of the sims you play is a one star celebrity (contagion effect by the way) and your dog is nice not aggressive he starts to be aggressive with the mailman/postman before he didn't care of him. I don't know why lol.
Celebrity is an illness you catch it by contagion. xD As if you speak to any celebrity you meet you become a celeb in your turn ahahahah.