Abu Dharr reported: Allah’s Apostle (may peace be upon him) said to me: “Don’t consider anything insignificant out of good things even if it is that you meet your brother with a cheerful countenance.” (Reported by Sahih Muslim)
The Prophet of Allah said: “Smiling in front of your brother is a charity.” (directed by At-Timidhi)
Smiling indicates that people love each other which make them like to deal with each other and talk with each other. This is so, we will always love each other as if we are at a heart of one man and one body as the Prophet of Allah (peace be upon him) describes us (Muslims) in the following Hadith. Nu’man b. Bashir reported Allah’s Messenger (may peace be upon him) as saying: “The similitude of believers in regard to mutual love, affection, fellow feeling is that of one body; when any limb of it aches, the whole body aches, because of sleeplessness and fever.” (Reported by Sahih Muslim).
Results of the smiling in front of others are gaining the reward for charity as well as the love of Muslims.