First, I don't think most people who answered this either read the question or they did not understand it.
The question is not, What makes you smile? It is, what makes you smile like NOTHING else can? It implies this is something, one thing that makes you smile the most or makes you smile the biggest.
Answers like the sun on my face or anything or bunnies and puppy dogs are not accurate answers to the question unless, seriously, the sun shining on your face makes you smile more than anything else on this planet.
Please read and comprehend the question before shooting off some fluff answer.
My mother. She has stood by my side when everyone else left. She stood by my side when she should have left. I don't smile often but when I do it is because of her.