My take on this is that culturally it is bad for a Thai person to show strong feelings in public. This I believe stems from the majority of Buddhists and Buddhist teaching that permeates Thai society. It is the same reason that kissing on public is frowned on.
If you express open anger or sadness, you can cause bad feelings in other people. And while it is bad enough to feel strong emotion yourself, it is much worse to cause that in other people.
Far better then to smile, whatever the circumstance, and do your best to propagate good karma to all.
Personally, I find the ability of Thais to put the feeling of others above their own utterly admirable.