Smile! It's sunnah

The magical effect that a smile has on others is well-known. Allaah The Almighty created mankind with an innate inclination to love those who are friendly. A person who meets others with a smile drives away their anxiety and troubles and spreads tranquility and comfort. This is because smiling is a commendable characteristic, and the one who smiles is complimented.
The Prophet, sallallaahu ‘alayhi wa sallam, was known to have a pleasant smiling face. In fact, it became a part of his character. The people who socialized with him recognized this characteristic in him. ‘Abdullaah ibnul-Haarith ibn Hazm, may Allaah be pleased with him, said: "I never saw anyone smile more than the Prophet, sallallaahu ‘alayhi wa sallam." [At-Tirmithi]
Jareer ibn ‘Abdullaah Al-Bajali, may Allaah be pleased with him, said: "The Prophet, sallallaahu ‘alayhi wa sallam, did not keep himself from me ever since I embraced Islam, and whenever he saw me he received me with a smile." [Al-Bukhaari and Muslim]
This was how the Prophet, sallallaahu ‘alayhi wa sallam, gained the affection of people, and this made them more ready to accept the truth that he proclaimed.
This was stressed by the saying of ‘Aa'ishah, may Allaah be pleased with her, "I never saw the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, laughing to the extent that I could see his uvula (the fleshy lobe at the back of his palate) - he would only smile." [Al-Bukhaari and Muslim]
This was further accentuated by the description of ‘Ali, may Allaah be pleased with him, "The laughter of the Prophet, sallallaahu ‘alayhi wa sallam, was mostly smiling; it was then that his white teeth showed."
Based on the previous saying, we can understand another saying of Jaabir in Samurah, may Allaah be pleased with him, who said,"The Prophet, sallallaahu ‘alayhi wa sallam, would remain silent for long periods of time, and would never laugh excessively."
Commenting on this matter, Imaam Ibn Hajar, may Allaah have mercy upon him, said, "What is understood from these Hadeeths is that the Prophet, sallallaahu ‘alayhi wa sallam, would, in most cases, only smile, and he would sometimes laugh. It is unfavorable to laugh too much or to exaggerate in laughter because it removes solemnity."
Books on the biography of the Prophet, sallallaahu ‘alayhi wa sallam, mention many situations in which the Prophet, sallallaahu ‘alayhi wa sallam, smiled. He, sallallaahu `alayhi wa sallam, would address those who were around him, issue Islamic verdicts or undergo different incidents, but he would face all this with a shining soul and a cheerful spirit. The following are some examples:
Anas ibn Maalik, may Allaah be pleased with him, said,
Once, while the Prophet, sallallaahu ‘alayhi wa sallam, was delivering the Friday Khutbah, the people stood up and called out, “O Messenger of Allaah, there is no rain (i.e. we are suffering a drought), the trees have dried up and the livestock are destroyed; ask Allaah The Almighty to bless us with rain.” So he, sallallaahu ‘alayhi wa sallam, said twice:"O Allaah, Bless us with rain." Then, a cloud appeared and it started raining. The Prophet, sallallaahu ‘alayhi wa sallam, then descended from the pulpit and performed prayer.
When he left, it was raining, and it rained continuously till the following Friday. When the Prophet, sallallaahu ‘alayhi wa sallam, started delivering the Friday Khutbah, the people started calling out, “The houses have collapsed and the roads are cut off; so ask Allaah to withhold the rain.” So the Prophet, sallallaahu ‘alayhi wa sallam, smiled and said:"O Allaah! Keep it around us and not on us."Whichever side the Prophet directed his hand to, the clouds dispersed and kept raining around Madeenah as a miracle of the Prophet, sallallaahu ‘alayhi wa sallam, and as an answer to his supplication. [Al-Bukhaari and Muslim]
Once, Suhayb ibn Sinaan, may Allaah be pleased with him, went to the Prophet, sallallaahu ‘alayhi wa sallam, while he had dates and bread in his hand, so the Prophet, sallallaahu ‘alayhi wa sallam, invited him to eat with him. When he started eating the dates, the Prophet, sallallaahu ‘alayhi wa sallam, looked at Suhayb ibn Sinaan, may Allaah be pleased with him, and remarked: "You have a sore eye!"It was known among the Arabs that eating dates aggravates such a condition. Suhayb, may Allaah be pleased with him, replied, "O Messenger of Allaah! I am eating [the date] from the other side [of my mouth that is under the well eye]." At that, the Prophet, sallallaahu ‘alayhi wa sallam, smiled. [Ahmad]
In another Hadeeth, Zayd ibn Arqam, may Allaah be pleased with him, said, "I once felt extreme anxiety and stress that no one else has ever felt. When I was walking with the Prophet, sallallaahu ‘alayhi wa sallam, I lowered my head out of distress. Then, the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, came up to me, tweaked my ear, then smiled at me. What the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, did was more beloved to me than having the pleasures of paradise on earth." [At-Tirmithi]
Also, Jaabir, may Allaah be pleased with him, said, "A man came to the Prophet, sallallaahu ‘alayhi wa sallam, and said: 'O Messenger of Allaah! I saw in a dream that my head was cut off.' Thereupon, the Prophet, sallallaahu ‘alayhi wa sallam, laughed and said: ‘Do not tell anyone if the devil tampers with your dreams.’" [Muslim]
‘Aa'ishah, may Allaah be pleased with her, said, "One morning, Hafsah, may Allaah be pleased with her, and I intended to do a voluntary fast. However, we received a gift of food and ate. When the Prophet, sallallaahu ‘alayhi wa sallam, entered, Hafsah, may Allaah be pleased with her, asked him before I did,‘O Messenger of Allaah! We intended to fast, but received food as a gift and ate some of it.’ Thereupon, the Prophet, sallallaahu ‘alayhi wa sallam, smiled, and ordered them to fast another day instead of that one.” [Al-Bayhaqi]
Another Hadeeth in this context is narrated by Abu Hurayrah, may Allaah be pleased with him, who said,
While we were sitting in the house of the Prophet, sallallaahu ‘alayhi wa sallam, a man came to him, and said: “O Messenger of Allaah, I am ruined!”
The Prophet, sallallaahu ‘alayhi wa sallam, asked:“What is the matter?” 
He replied: “I had sexual intercourse with my wife while fasting.”
The Messenger, sallallaahu ‘alayhi wa sallam, asked him: "Can you afford to free a slave?"He replied in the negative.
The Prophet, sallallaahu ‘alayhi wa sallam, asked him: "Can you fast for two successive months?" He again replied in the negative. 
The Prophet, sallallaahu ‘alayhi wa sallam, then asked him: "Can you afford to feed sixty poor persons?"He replied in the negative.
After a while, some dates were brought to the Prophet, sallallaahu ‘alayhi wa sallam, then he asked:"Where is the questioner?" He replied: "I [am here]."
The Prophet, sallallaahu ‘alayhi wa sallam, said [to him]:"Take these [dates] and give them in charity."
The man then said: "Is there a person poorer than me that I can give it to? By Allaah, there is no family between the two mountains [of Madeenah] who are poorer than me."
The Prophet, sallallaahu ‘alayhi wa sallam, smiled till his pre-molar teeth became visible, and said:"Feed it to your family."  [Al-Bukhaari and Muslim]
The advice of the Prophet, sallallaahu ‘alayhi wa sallam, is sufficient for us: "To smile at your brother is charity." [Ibn Hibbaan]