This is because she likes you. Obviously, it is very important for her to let you sing a birthday song to her. Such an important day shows that your blessing to her is very important to her. Because she likes a person, his every move is very important in the other person's eyes. If she doesn't care about you, how can she care about a word you say? If she likes her, go after her. If she doesn't like it, don't give the other person the wrong feeling. Hope to adopt!