why people smile

Phew..Finally am flying home :) last few days were really llllooooonnnngggg. I never thought travelling alone to US can be so tiresome.
Not boring though, I never get bored ..
So here Iam at Heathrow wating for my flight to Bangalore with more than six hours of transit time What do I write about..Hmm..perhaps about the most important observation of this trip :p .
Observation:
Whenver an European/American comes out of a lavatory in a plane he smiles at the next person in queue.
But I have observed that the asians were not quite comfortable in doing it.
Though there is nothing wrong in exchaning a pleasant smile, but why while coming out of a lavatory ?
There were three different kinds of smiles and here are the hidden reasons behind those smiles

a.Big Relief smile - Hope u understand how happy u feel to let it all go after patiently waiting in a long queue with a conscious and hard effort of not polluting the env and napalming people in the flight and hence the smile.
b.Save me smile - Some people did it to save the embarassement. May be they were embarassed that the people waiting outside might have heard all the strange noises that they made inside, may be they thought their pugnacious substance leaked out of the lavatory..whatever..hence the smile
c.Killer smile - I observed that very few people gave this smile, this cannot be classified as smile but something like a villainous grin. It conveyed this, just go inside, this is how hell smells. Enjoy the ma wondefrful aroma..

But why dint the asians smile? May be they don’t give a shit about anything..