Emoticons help to show emotions through the text, so why not let everyone know that you are having a good day?
Wearing this t-shirt might help you avoid more children running away from you to their parents screaming 'this scary man was looking at me!'. In case anyone asks why are you not smiling, you can just point them at your t-shirt and show that you, indeed, are smiling, so they can now leave you alone!
What not to smile about :D?!