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Brainstorm your answers. In other words, write down everything that comes to mind. This could just be a phase of different interests, and that is entirely okay.  Things like "It makes me happy" are great but try to ask yourself why it makes you happy. Write it down. Some reasons that are important to you, like "I have a deep psychological need for approval and appearing on a  stage can meet" may be a great reason (and is), but probably not the best bet for talking to your folks.
Ask yourself why you want to be an actor. Analyze the list for good reasons, like "I'm good at it" or "My friends tell me I should try". Be specific. Edit your list on a new sheet of paper.