The noble scholar, Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah stated:
And it is befitting for the student of knowledge that he be a person of beautiful manners and temperament, constantly (a person) of happiness, constantly (a person) of relaxedness – because (this) was from the characteristics of the Prophet sall Allaahu ‘alaihi wa sallam which Allaah created to be in his natural disposition and with which He blessed him.
And He said about him:
And indeed you (Muhammad) are of exalted manners, the manners of the Qur-aan.
Soorah al Qalam (68) aayah 4
From his characteristics was that he was constantly (a person) of joy, plentiful in smiling – may Allaah’s salaat and salaam be upon him.
And there is no doubt that if a person is always joyful and plentiful in smiling then the person who meets him will feel happiness from (meeting) him.
But if you were to meet a man, frowning and gloomy, would you be happy due to meeting him? No. Therefore my brother, be always happy, plentiful in smiling, with a bright and cheerful face.
And therefore from the characteristics of Eemaan is that you meet your brother with a cheerful face. And know with the knowledge of certainty that if you possess this characteristic, you will be a person who is calm and relaxed, (both when) standing or sitting, in your house and in the market. And be constantly happy with your brothers, with a cheerful face and plentiful in smiling.
(Quoted in Wasaayaa wa tawjeehaat li tullaab il ilm p49 of Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah, translated by Nasser ibn Najam)
 Refer to Tafseer at Tabaree