I was thinking about this yesterday when a friend had some good news to share and I was anything BUT in a congratulatory mood. Still, if the tables were turned and I was in her shoes, I would certainly want (and expect) people to congratulate me. So I tucked in my frayed emotions and put on my "happy face" and offered up my most sincere-sounding congratulations.
Does that make me a fake, mask-wearing fraud? I don't think so. As much as we are coached these days to just "be ourselves", I think it's also important to know where to draw the line between TOTAL HONESTY and TACT. While we all need to make sure that we have a safe outlet (a good friend, a diary) for our emotions, there is no reason why we must constantly be an open book. It's OK to keep some things to ourselves, especially when wearing them out in the open might darken someone else's day.
But beyond just knowing how much of ourselves is appropriate to show the world, there is a certain truth to the phrase, "If you tell yourself something often enough, you will start to believe it". This can certainly go both ways; but I think it can be a positive thing, if you find yourself constantly bogged down by negative thoughts and decide to "combat" them by forcing yourself to be happy on the "outside". Eventually, after hearing yourself often enough, you may start to feel like things aren't so bad after all.
This takes us back to the original question though...we, of course, want to be genuine in the way we interact with others and so I have found it helpful to learn to separate my "dark side" from my "light side", if that makes sense. The mind is an incredible thing and has an immeasurable capacity for storing all sorts of thought and emotions. Just because you don't "feel" happy, doesn't mean that happy thoughts aren't there and that you aren't being sincere when you express them to others. But if you have been preoccupied lately with unhappy thoughts, it may just take a little digging before you are able to locate the positive/optimistic part of your brain.
I am no scientist or psychologist. I only know what I have experienced and recorded from my life experiences in my personal journals. Sometimes a mask is a good thing. Just don't keep it on too long.