Because she was mentally and physically abused and raped as a child and never fully recovered because instead of getting her mental help and arresting the man/her father who did that to her the police just drop her off at her grandmothers.
He does it because other than family she was the only one who treated him like just a regular kid and she had his back the whole time they were growing up. Plus he was raised by a good mother who raised him to be a good man.
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