Most people would say this cat, Boomer is mad about something. Doesn't matter what really, but it looks like he's going to say, "If you don't stop RIGHT NOW, I will bite you!"
We learn to read faces with experience. Temple Grandin writes that Ausperger people typically focus on the mouth, not the eyes when reading faces. It has to do with paying attention so hard to what is being said because of listening challenges, that they miss other cues. Regular people know to look at the eyes of a face first & factor in what the rest of the face says. This is teachable, but if you don't understand the features of each emotion, its extra tough.
So, look again, see Boomer's pretty smile & nice teeth, its kinda funny looking isn't it, makes me smile, such an interesting face. See how his eyes crinkle, its a big smile. See his ears tip like he's going to do something fun? OWWW!!! I just got bit. How did that happen?