Imam Assim Al-Hakeem Explains why women are not permitted to smile at men. This is in keeping with Islam’s mandate to segregate men and women.
When a woman smiles at a man what is this? Even disbelievers say that’s an invitation. So, it is an invitation when she smiles regardless of her age, regardless of her beauty, regardless of her social status, married or single. If you look at the vast majority of evidence in the Quran and the Sunnah, you will find that this religion is based on segregation between men and women. To the extent that clear evidence in the Quran that prohibit women from revealing their beauty.
A woman who leaves her home wearing her perfume and men smell her perfume she is like an adulteress. It is better to be stabbed in the head with a needle of iron than to touch a woman that is not halal for him.
Islam promotes and endorses the segregation between men and women for the benefit of men and women. So, after all of this no one in his right mind would say what’s wrong in women smiling. Smiling is a clear invitation, and it defies the purpose of hijab.