Here's why:
* Her father is missing: This is the most obvious source of worry, but it also fuels her feelings of inadequacy. Meg feels responsible for her father's disappearance and believes she could have done more to help him.
* Her brother Charles Wallace is different: Charles Wallace is a gifted child, but he is also very different from other children. He's intelligent beyond his years and can be socially awkward. Meg worries about him and feels protective of him.
* She feels like an outsider: Meg struggles to fit in at school. She's not popular and doesn't feel like she belongs. This makes her feel even more isolated and alone.
The combination of these factors makes Meg feel overwhelmed and inadequate. She yearns for a way to be different and be accepted, which is why she is so excited when Mrs. Whatsit appears and offers them a way to find her father.