A lot of people, when not socializing, will look rather angry due to the fact that they are deep in thought. A lot of introverts are like this. I smile a lot when I'm talking to someone, but if they look at me from a distance, I look like I'm gonna punch someone in the face. It's just because I'm deep in thought.
Judging by the fact that most people on the forums are introverted, most here probably have angry or serious neutral faces.
Also, it is generally considered socially acceptable to have a slight smile on your face at all times. So when someone does not display this, they are already singled out as being "not happy."