I think the reason why she shouldn't dance on the subway is:
1. The subway belongs to a public place, so you should pay attention to your words and deeds in public places. The subway is not someone's private place. Pay attention to your behavior on the subway. The subway is everyone's. If someone dances on the subway, it will affect others more or less. Many people use subway time to rest, and dancing will affect their rest. If you want to dance, you can go to the square, which is a place for public activities.
Dancing on the subway is not safe. The subway is also completely stable during its operation. If there is a little bump, the dancer is likely to get hurt. When the elder sister danced, there were people beside her. If the subway is bumpy, she is likely to fall on others, so others may be injured because of her.
3. Consider other people's feelings in public places. When we are in public places, we should also consider the feelings of others, regardless of whether this sister's dance is very beautiful, even dancers dancing on the subway will not let everyone like it. You are willing to jump, but others may not be willing to watch, so consider everyone's feelings before doing things in public places. Many people like the quieter subway.
Although the elder sister's approach is somewhat inappropriate, the optimistic attitude towards life is also worth learning, but the environment should also be considered next time. No one will say anything if this matter is put in the square, but the subway belongs to everyone after all. Pay attention to your words and deeds in public places, not all actions will be accepted by everyone.