stay normal, not so happy!

i recently came accross a term referred to as " default emotion".

on thinking about it a bit i concluded that it must be what generally remains on the faces of people unless something special (that may be either good or bad) has happened with them at any given point of time.

on getting deeper into thinking about it, i noticed that the expression that can generally be seen on the faces of people around is either that of absolute indifference, or, that of unprovoked anger.

i am still wondering why carrying a pleasing (kind of smiling..) expression is considered to be abnormal or somehow, just strange?

is'nt it so that if you just walk into your school/college/workplace/marketplace, etc. with a simple relaxed and pleased look you are most likely to be asked by the people around that " hey! what's the matter, anything special?" you may even get a few surprised stares on flashing such an expression.

however, if you walk around with an expression where your mouth is closed with a very little bit of extra pressure on both the lips to make them remain joined together and taking due care (subconciously though) that they are not anyhow forming into a smile like curve, a look in the eyes as if they are continuously in search of a known "target" , and the firm straightness of the head (sometimes even a very minute backward tilt), in that case, nobody would least bother to ask you - hey! what's the matter, looking quiet indifferent?

i have heard and read that a number of human habits/instincts have been formed as a concequence of the experiences of the early humans in primitive ages.can this "default emotion" also be a part of such a development?or is that not "development" but "advancement"?