“Do not look down on any act of kindness, even if it is just meeting your brother with a pleasant smile on your face.” [Sahih Muslim]
Ibn ‘Uthaymeen said:
“A pleasant smile causes people to become happy, wins over their love and affection, and leads to both parties having good thoughts of each other. Just try it and you will see.
However, were you to frown upon the people, they would stay away from you and not feel comfortable sitting with you nor talking to you.
You may even become afflicted with a dangerous condition: high blood pressure. Having a tranquil chest and a pleasant smile is the most wholesome remedy and the best preventative measure for this condition. For this reason, physicians advice people who have this condition to stay away from what disturbs and angers them, since those things only agitate this condition.
A pleasant smile actually has a remedial effect on this condition, since it leads to a tranquil chest, and it causes others to love you.”
[Taken from: “Upright Moral Character”, by Ibn ‘Uthaymeen, p. 49]
((copied from blog – http://salaf-us-saalih.com/2012/06/27/upright-moral-character-the-effects-of-a-pleasant-smile/ ))