I prefer not to show or discuss my emotions if I can help it. I prefer to have simple, straightforward conversations with people, and emotions...complicate things.
As an example, if I'm angry or upset, people usually feel obliged to ask why, and the reality is that they honestly don't care and I really don't want to explain why when it's just going to lead to an awkward situation as neither of us will have any idea what to say, so the simple answer is not to let on what I'm feeling: that way other people can remain blissfully ignorant - or at least pretend to be so without feeling awkward - and I'm spared the difficulty of having to explain and the social guilt for feeling something that others don't really want to see or know about.
That can apply for positive feelings as well as negative ones, so I tend to keep those bottled up as well...happiness isn't as contagious as people like to think, and contrary to popular belief misery usually just wants to be left alone.
I'm not particularly comfortable displaying my emotions regardless of what they are, and really they're nobody's business but my own. Unless things become unbearable or I'm practically skipping with delight I like to put on an amicable and (hopefully) approachable front and not let my emotions get in the way of life in general. Emotions are a private inconvenience best kept private, at least for me.