Personal Development Attribute 

A smile: "form one's features into a pleased, kind, or amused expression, typically with the corners of the mouth turned up and the front teeth exposed." 

Reflection: When you smile at others you can make there whole day a whole lot better. In this quote it is saying that when you smile, you are changing the world by showing how positive you are. In the second part of the quote i believe it means, do not let others put you down or make your smile turn into a frown. Basically do not let anyone mess up your day. The way you see the world should not determine weather you are happy or not. You can make yourself a happy person. Do not let the negativity and struggles in life change your smile. 

Example: If there is a new student in school and he or she looks scared or lost, the first thing you can do is smile at them. This will help them 100% more , they will give you a smile back and you  will become there friend with one smile.