Men and women are equal in many aspects such as creation from a single essence and enjoying rights to education, freedom, speech, expression, etc.
We should also note that men and women are different in their physical and psychological make-up. These differences lead to the creation and enactment of specific rules for women such as the mandatory observance of Islamic modest dress in society. Woman is the expression of beauty and attraction; Men are enamored by women and will do most anything to win them over and to impress them. Obviously, women (not men) should be told to avoid exposing their adornments to men.
Of course, it should be noted that it is not true to say that modest dress is specific to women and that men are not under the obligation to observe Islamic modest dress.
For further clarification of the answer, it is necessary to go through the following points:
1. Men and women are equal in many aspects such as creation from a single essence and enjoying rights to education, freedom, speech, expression, etc.
2. Despite the fact that men and women are equal in many ways, they are also different from each other in some aspects. Late Allamah Tabatabai says, "But while sharing these basic qualities with man, she differs from him in other ways. An average woman lags behind an average man in the build of her body and its basic organs, like the brain, the heart, the veins, the nerves, her height and weight. (The details may be seen in any book of anatomy.) As a result, her body is comparatively soft and elegant, while a man's is tough and rough. And the fine sentiments, like love, tender -heartedness, and inclination towards beauty and adornment are more pronounced in her than in man. On the other hand, the reasoning power is more prominent in man than in woman. The woman lives a sentimental life; the man an intellectual one. It was for this reason, that Islam differentiated between men and women in those duties and responsibilities which were related to reason and those related to sentiment. Ruling, judging and fighting have been reserved for man, because these things are closely related to reasoning and thinking. And the bringing up of, and looking after, the children, the domestic management has been reserved for woman.
3. Modest covering is not a duty of men only; it is also necessary for men, to some extent, to wear modest dress.
4. The philosophy of the Islamic 'covering' depends on several things. Some of them are psychological and some relate to the home and the family. Others have sociological roots and some of them relate to raising the dignity of a woman and preventing her debasement.
A) Psychologically, Islamic covering helps promote psychological wellbeing.
B) Islamic covering solidify the family relationship and helps in maintaining complete sincerity between the husband and wife.
C) Sociologically, the observance of Islamic modest dress leads to perseverance of society and restoration of work and social activity.
In other words, the modest dress in Islam is rooted in a more general and basic issue. That is, Islamic precepts aim at limiting all kinds of sexual enjoyment to the family and the marital environment within the bounds of marriage so that society is only a place for work and activity. It is opposite of the Western system of the present era which mixes work with sexual enjoyment. Islam separates these two environments completely.
5. The reason behind the necessity of Islamic modest dress for women is that they are the expression of beauty and attraction; men are enamored by them and will do most anything to win them over and to impress them. Obviously, women (not men) should be told not to expose themselves and their adornments to men. Therefore, in spite of the fact that men have not been commanded to cover themselves (like women), yet when they go out they cover themselves more appropriately than women do. That is because men tend to look or gaze at women. They do not like flirting and showing off as women do. The contrary is true with women; they like to show off and expose their adornments rather than to look at men.
The inclination existing among men to look at women further stimulates a woman to reveal her adornments. It is for the same reason that tabarroj (revealing adornment) is particular to women.
The concluding point is that Islamic modest dress causes a woman's dignity and respect to grow in man's eye. The fact that a woman considers and maintains privacy between herself and a man is a secret tool which she has been using to preserve her position and status vis-à-vis the men. Islam has encouraged woman to use this tool. If Islam has emphasized on women to walk modestly and not to expose her adornments to women, it is all because it wants women's dignity and respect to be preserved and that hey should not be regarded as a worthless chattel that could be easily bargained by others.
In other words, a woman's being clad in modest dress within the limits and boundary set by the Islamic law will give her a more dignified and respectful place in society because it protects her from the aggression and abuse of people with bad character. Hence, therefore, modest dress (hijab) is not limitation; rather, it is immunity and protection. Not only does it provide immunity to women but it also immunizes the whole society against sins.
For further information see:
1. Ayatollah Ahmad Mohseni Gurgani, Durr-e Naab dar Sadaf-e Hijab, Islamic Sciences Publications
2. Muhammadi Ishtihardi, Muhammad, Hijab, Indicator of Personality, 1st edition, NAJA Publications.
3. Razzaqi, Ahmad, The Factors behind Corruption and Immodesty and Methods of Combat against It, 4rth edition, Islamic Publications Organization Publications, 1992.
4. Shuja'ei, Muhammad, Durr wa Sadaf, 3rd Edition, Publication 1999.
5. Mutahhari, Murteza, The Islamic Modest Dress, Sadra Publications
6. Qira'ati, Mohsen, The Islamic Covering in Islam, 9th edition, Naser Publication, 1991
7. Javadi Amuli, Woman in the Mirror of Glory and Perfection, 2nd edition, Raja Cultural Publications, 1992.
8. Fattahi Zadeh, Fathiyah, The Islamic Modest Dress according to Quran and Sunnah (Tradition), 2nd edition, Islamic Propagations Office Publications 1997.
 - Translation of Tafsir Al-Mizan, vol.2, pg. 416
 - For example, it is necessary for him to cover his private parts. Covering the rest of her body is also necessary, if it causes others to fall into a sin or if it entails evil except for those parts which are not normally covered like the head and neck. Current Legal Issues, vol.3, pg. 227 and 228.
 - Mutahhari, Murteza, The Islamic Modest Dress (Collection of Works), vol.19, pg. 432
 - Mutahhari, Murteza, Collection of Works, vol. 19, The Islamic Modest Dress, On the Command to Guard their Parts.
 - Vide: Ibid, with little modification.