I like a girl and she knows that I like her. Every time I send a text message to care about her, she just replies with thanks. Why?

If you like a girl, she knows that you like him, but every time you send a text message to care about him, he just says thank you. I think such girls are very polite. He only replied with a thank you. Maybe he just thought it was enough for a friend. Say something else, so that he can't call friends any more. So he just says thank you every time, because he may not like your words that much. Maybe it's just a little feeling He didn't think you needed to be so advanced, so he answered one casually. Thank you.