The Glass Kingdom was in danger because of a monster, so they requested that you save them. Marceline decides to tag along with you. You were struggling to save the glass people, and it didn't help that Marceline kept trying to get your attention.   This causes you both to argue because she didn't care what happened to the glass people as long as she had your attention. After you argued, she humiliated you with a song full of hurtful insults directed at you. Her singing scared the monster away, saving the glass people in the process. She looks away from you. "There problem solved."