The primary reason that women wear modest clothing is because the practice was ordered by God and conveyed through the Qur'an.God says (interpretation of meaning) -
And say to the believing women that they should lower their gaze and guard their modesty; that they should not display their beauty and ornaments except what (must ordinarily) appear thereof; that they should draw their veils over their bosoms and not display their beauty except to their husbands, their fathers, their husband's fathers, their sons, their husbands' sons, their brothers or their brothers' sons, or their sisters' sons, or their women, or the slaves whom their right hands possess, or male servants free of physical needs, or small children who have no sense of the shame of sex; and that they should not strike their feet in order to draw attention to their hidden ornaments. And O ye Believers! turn ye all together towards Allah, that ye may attain Bliss. [Quran 24:31]
The actual and absolute reason is utter and complete submission to Allah [The One True God]. He commanded. Why wouldn't I obey God? He ALLAH, gave me understanding. I will obey Him. I don't see anything else!
And I will express more. Its dedication to Allah.
And I will express more. You reminded me of the first time I took this decision [8 years ago]! Its like experiencing it again!
I am saying I am wearing it for Him Allah, although i have very little knowledge about Him, but i sure do know a little basics! He is worthy of Worship!
I wouldn't wear it for praise because my classmates get mocked for wearing it!
I wouldn't wear it for fashion, cause this is not at all IN fashion today.
I don't know why I took this decision, where as I always had been a very shaky person! My brother changed his path, he hasn't forced me to wear it, at least not in the past 2 years. But I want to wear it, because i wanted to when i was just 7-8 years old. I thought this hijab will make me INDEPENDENT, will make ME STRONG, MODEST.
So I wore it, not knowing for how long. But I sure would pray to Allah in the past for being righteous. Maybe this is the guidance?
Suddenly people are different around me, they changed their behavior in a span of day and night! I could feel my best friend was ashamed of me, asked me to lie to people, that i had gone for Pilgrimage or its a fashion. I rebelled at her first time. i don't know what got into me, i felt strong, defending. which was not my character.
School passed by, the atheist head-boy of my school asked for how long? I replied FOREVER.
My whole life took a new road! Everything changed around me. There's good around me, many don't have that. I am very Fortunate. Hijab was one small part that opened my gate to Islam.