Allah Almighty tells us about when Prophet Mohammad pbuh was with one of the leaders of Quraish preaching to him and hoping he will embrace Islam. While he was talking to him, Ibn Umm Maktum, a blind man who had converted to Islam a while back, came to him to ask him about a matter and persisted in his inquiry. The Prophet wished that Ibn Umm Maktum would go away so he could continue his talk with the other man, hoping to guide him. He turned away from Ibn Umm Maktum and frowned, and moved back to the other man. Even though the blind man could not see his face, Allah revealed these verses reproaching His noble Messenger to emphasize the absolute importance of giving full and pleasant attention to seekers of truth.

(He frowned and turned his attention away, simply because the blind man came to him interrupting. You never know O Prophet, perhaps he may be purified, or he may be mindful, benefitting from the reminder. As for the one who was indifferent, you gave him your undivided attention, even though you are not to blame if he would not be purified. But as for the one who came to you, eager to learn, being in awe of Allah, you were inattentive to him) Surah Abasa 80: 1 to 10.
الله سبحانه عاتب نبيه محمد صلى الله عليه وسلم لأنه عندما كان يوما يخاطب أحد سادة قريش، وقد طمع في إسلامه، إذ أقبل إليه ابن أم مكتوم، وهو رجل أعمى وكان أسلم قديما، وقد جاء إلى النبي ليسأله عن شيء وألح على النبي في السؤال. وود النبي أن لو كف ابن أم مكتوم عن سؤاله ليتمكن من متابعة حديثه مع ذلك الرجل طمعا في هدايته. وعبس في وجه ابن أم مكتوم وأقبل على الآخر.

ورغم أن الرجل الأعمى لا يرى إلا أن الله عاتب رسوله الكريم وأنزل الله تعالى هذه الآيات معاتبا: (عَبَسَ وَتَوَلَّىٰ (١) أَن جَاءَهُ الْأَعْمَىٰ (٢) وَمَا يُدْرِيكَ لَعَلَّهُ يَزَّكَّىٰ (٣) أَوْ يَذَّكَّرُ فَتَنفَعَهُ الذِّكْرَىٰ (٤) أَمَّا مَنِ اسْتَغْنَىٰ (٥) فَأَنتَ لَهُ تَصَدَّىٰ (٦) وَمَا عَلَيْكَ أَلَّا يَزَّكَّىٰ (٧) وَأَمَّا مَن جَاءَكَ يَسْعَىٰ (٨) وَهُوَ يَخْشَىٰ (٩) فَأَنتَ عَنْهُ تَلَهَّىٰ (١٠)) عبس 80: 1 إلى 10.

It is highly commendable for a Muslim to smile at his Muslim brother. The Prophet pbuh said: "Do not belittle any good deed, even meeting your brother with a cheerful face." Cheerful means to be pleasant and smiling, so that the person would feel comfortable. Through this, Muslims' hearts will be brought together.

The Prophet pbuh also stated: "Smiling in the face of your Muslim brother is considered a sadaqa." When you are kind to people, you are also kind to yourself. The Muslim is acting kindly to himself when people think of him as a pleasant man rather than a grumpy one, earning divine rewards and social love simultaneously.
يستحب للمسلم التبسم في وجه أخيه المسلم، كما قال النبي صلى الله عليه وسلم: (لا تحقرنَّ من المعروف شيئًا، ولو أن تلقى أخاك بوجه طَلْق). ومعنى طلق: ضاحك مستبشر، وذلك لما فيه من إيناس الأخ المؤمن، ودفع الإيحاش عنه، وجبر خاطره، وبذلك يحصل التَّأليف المطلوب بين المؤمنين.

ومنه قوله: (تبسُّمك في وجه أخيك لك صدقة)، أي: إحسان إليه، أو إحسان لنفسك لأن فيه ثواب الصدقة، كما أن المسلم يحسن لنفسه حين يذكر بالابتسامة بين الخلق ولا يذكر بالعبوس.

When meeting non Muslims, one should also be pleasant. This is especially true if you want to call people to Islam by either talking to them or by setting an excellent behavioral example. By smiling and being consistently pleasant, you will make people accept you, and by extension, they will become more receptive to the beautiful message of Islam.
أما مقابلة غير المسلم فإنه لأجل دعوته للإسلام بالكلمة أو بالسلوك فإنه يقابل بالتبسم والبشاشة رغبة في دعوته إلى الإسلام وتحبيب الإسلام إليه.