Why did I get rejected?

I am an international student, I met a girl in my hall, the first day we talked for about 2-3 hours and I was the one to end the talk, Second Day she started asking me the girls I like and if I have a girlfriend, we started spending most of our time with each other, We Used to eat together, Used to talk until 3-4 am, used to spend 2-3 hours of private time just talking to each other on the roof at night. She started complimenting my fragrance, she started spending most of her time in my room even tho I never called her. On the 5th day, We were all good, she left my room at around 12:00 am. my brother persuaded me to propose to her (though I did not even think about it on my own). I called her on the roof and said "I like You", after 5 minutes of being confused and in shock, she said "NO".

I am still wondering why she said no even though she gave all signs of liking me, did I come as a jerk as we were together for only 5 days and have a totally different ethnic background.
Why did I get rejected?
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