Does not smiling make you depressed?

My mum's friends have this weird idea that I am depressed simply because I do not smile when around them.

The reason for this being they do not give me anything to smile about. And nobody randomly sits there smiling at nothing.

So now to take the piss, I purposely don't smile when I am around them.

But I am still trying to get my head around their logic but I cannot therefore today's question is...

Question : Does not smiling make you depressed?

Strange Logic

Why do the speedometers on an average car go up to 120mph when legally you can't drive THAT fast on any road?

If an orange is orange, why isn't a banana called 'a yellow?'

And why did the person or people who named everything name it as they did?

Question: Can anyone answer those questions or is it going to be unanswered forever :S