Struggling with Modesty in Summer

Assalamu alaikum, I'm having a tough time with modesty. In summer, when we go out, my friends wear light, short clothes, and I feel so out of place. When they ask why I cover up, I don't know what to say. They tell me the human body isn't immodest by itself, and now I'm confused about why we must dress modestly. I need help understanding.