Everyone knows it's when I'm smiling you really need to worry, and here's why.
If I'm angry, or upset, sure I might do something terrible. Because at that moment in time I won't care about the repercussions. I've disassociated myself from my normal emotions on the matter. That's not to say the shock of those won't hit me later and even though I'll know why I committed those actions at the time, I may actually come to feel guilty about them once whatever blinding extreme emotional state has passed.
If I'm smiling, that's the dangerous time, because then I am acting out of calculated premediated thought. I have considered the consequences of my actions, I am well aware of what I might bring down upon myself or those around me. I have weighed them in the balance and I have not found them wanting. I am perfectly settled on whatever course I have set me on and I have already prepared myself to face the fallout.
That is why just because I am smiling does not mean everything is okay, or you should ever let down your guard. When I smile I am acutely conscious of both myself and what is going on around me, and you forget that at your peril.