No one does anything for no reason. It could be they find the egocentric perspective of others humorous. It could be an involuntary muscle contraction caused by gas.
Most people respond to a smile with a smile.
it is not. you would have to say, "she passed me a smile" or, "she passed us a smile" and so forth.
When you smile at someone but you are not happy
The phrase, "a smile to die for," means that someone has a beautiful smile. If someone has told you this, then you should take it as a compliment.
Of course it is! Smile at someone and they (should) smile back!
YES!
Nothing
The phrase "put a smile on your dial" is a colloquial expression that means to make someone happy or bring a smile to their face. The term "dial" refers to a person's face, particularly their mouth where a smile would appear. So, when someone says they will "put a smile on your dial," they are promising to bring joy or happiness to you.
Most likely something from a South Park album...
I would smile and say Thank you and give them a hug.
if someone loves you then they would look at you or smile at you all the time. they would also tell you something he or she never told anyone like if the love you.
In Portuguese, you would say "Eu gosto do seu sorriso." This phrase expresses appreciation for someone's smile in a friendly and warm manner.