Can somebody tell me why I feel lonely when I am alone..

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I am lonely every time I ride going to my school because I am not with anybody even anyone of my classmates. I used to be with my girlfriend before. She has just arrived yesterday from far away. She has been there for one and a half year. I am not sure if we will be as the same as before. I just feel happy when I am with my best friend, friends and classmates. I have never been like this before. I used to be a happy person before. I hope there are lots of people who could help me.