Why do we feel that the default emotion is to be happy?
After examining the contents of our minds and the things hidden beneath the surface that we have carefully chosen not to surface - it will be apparent why we shouldn't aim to be happy. Trying hard to be happy when we are not feeling so is even more agonizing.
We should be given opportunities to feel the original emotion instead of "Don't worry, be happy". Here, what we have to do is its opposite - "Worry, try to be a bit sad and let the emotions take their natural expression". Initially uncomfortable is this activity but over time, we will develop a clarity in thought and an authentic self and a non-cynical attitude. Naturally we would develop a brilliance and cheerful attitude which will be long term, lasting the test of time in various situations of work, family, friendship and general life.
When a child feels sad, we should refrain from saying "What is there for you worry about", instead literally dropdown to their level and empathize with what they are feeling -- "Yes, it is true".
If our friend got rejected, we should refrain from saying "She is not suitable for you anyway", instead console with "that sound difficult".
Maybe it takes a few years, to wipe of the scowl face we have developed over our encounters that far in life. Note that we have some raw memories and circumstances which are not processed may be for a longer period of time (days to months to years). We can take the help of books, people who have gone through similar situations and learn to process the silos of information inside our heads by labelling what a situation actually mean, and if possible give each emotion a well crafted name such as melancholy, sadness, jovial, saudade, forelsket.
Of course, sadness doesn't look good on our face, neither does a suppressed one with a jovial expression. People do understand, at least we can hope. But, being fake we cannot guarantee whether they would stay along without a considerable friction. We tend to feel that being happy attracts people, that's fine. But, we will somehow drive them away if we keep unfinished business inside which naturally blurs our choices.
If we are feeling sad, let us do the courageous thing "Be sad". And remember that it will be only a matter of time, generally few days, before we feel fine once again.