[@Chaotic Chao] Not like she doesn't think she couldn't deal with it.

More like her mom was abused.

Her mom said she never wanted Mirei when she was arguing with her father.

Mirei heard on accident. (Her mom didn't think she was home) 

She thought she was loved but she was young and simple-minded so automatically thought this meant she wasn't loved.

Her mom committed suicide when Mirei was 13 and left a note saying "I love you Mirei" and Mirei blames herself for whatever. She thinks it was her fault.

Thinks that love is something someone makes up.

 More reasons but main ones are this. So yeah~