Why is my martial arts teacher so kind in class but silent when we’re alone?

When we are alone he doesn't talk, doesn't seem interested, when we are in class (Martial arts, him being the teacher, he's 22) he's really kind to all of us and I even thought he's specially kind to me, there was even a time when our other classmates whistle him once because he kept asking me to be his partner (it was like 3 times in activities) in clases while him being the teacher, or like last time a week ago, I showed that I really liked two songs from a band and for the entire class that bands songs sounded until there were no more. He speaks easily to others, even some girls but the dojo doesn't have that many girls, yet when we are alone he prefers being on his phone (I don't) and more than 4 months have already passed since I met him and nothing happens.

Why is my martial arts teacher so kind in class but silent when we’re alone?
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