When you come across someone who smiles and treats you kindly, you will no doubt be pleased and feel close to him or her. However, if he or she meets you with a frowning face, you would try to avoid him or her, even if there is benefit in meeting him or her.
Hence, the Islamic Shari'ah encourages Muslims to be cheerful and delightful, as it is reflected on one's face and expresses amiability and a desire to socialize. The Prophet, sallAllahu ‘alayhi wa sallam, considered meeting other Muslims in a jovial and happy mood, as a good deed, when he said: "Every good deed is a charity and it is a good act to meet your Muslim brother with a cheerful face." [At-Tirmithi] In another Hadeeth, he, sallAllahu ‘alayhi wa sallam, said:"Do not belittle any nice action; when you speak to your brother while smiling at him, it is a good deed [on your part]." [Abu Dawud, Sahih – Albani]
The Prophet, sallAllahu ‘alayhi wa sallam, himself was always pleasant and buoyant, even with hard-hearted and harsh people. Once, a Bedouin came to him, sallAllahu ‘alayhi wa sallam, and jerked his cloak violently off him, scratching him on his shoulder; he then harshly said: "O Muhammad, order that I should be given out of the wealth of Allah which is at your disposal." The Prophet, sallAllahu ‘alayhi wa sallam, turned to him and smiled, and directed that he should be given charity.
When one meets his Muslim brothers with a cheerful countenance, his love and affection in their hearts develops and makes them fond of meeting him. That is why some poets penned verses in this respect, indicating that when a man is smiling and jovial, it encourages others to visit him. Alternately, if he is not so, people will detest going to him, even if it serves their interest. They also say that cheerfulness toward guests should precede their other rights of being offered food and drink.
Undoubtedly, no one can afford satisfying all people materially; however, he or she can always win their love by smiling to them and exhibiting good manners. Even when people ask a favor of a Muslim, he or she should be in good spirits, as was the example of the Prophet, sallAllahu ‘alayhi wa sallam, who was never slow to serve Muslims and fulfill their needs with a cheerful face and a big heart.
Some verses of Arabic poetry praise generosity; perhaps, the Prophet, sallAllahu ‘alayhi wa sallam, is the one who warrants being described by them, as they sketch an image of a man who would give away everything that he owned without hesitation. They talk about how, even if someone takes something from such a person, he is jovial and happy, as if he is the one who was giving out of his own accord. It also poetically describes that his generosity is such that even if he had been asked for his soul, he would have parted with it. Yet, the poet cautions those who ask of him, sallAllahu ‘alayhi wa sallam, and others like him, to fear Allah The Almighty and not to take advantage of this noble characteristic.
Therefore, be cheerful, kind and lenient; always smile and avoid frowning. Indeed, a Muslim should meet people happily and treat them with good manners; he or she must also try not to argue with them, even if they ask for something that is impossible. In this way, one will win their affection, for he or she has abandoned dispute and conflict.- islamweb.net