Why do some people over react to stuff other people wouldn't really care about?

There's this girl from my school, lately we been talking more now that it's summer and schools over and what not. Anyway she was telling me about her life and how "hard" she had it, and how it eventually drove her to a depression. She started getting over her depression at about my age, and now she's still working on getting over it but she's pretty much a lot better. Well we actually have a lot in common, minus the depression. Anyway her sob story goes, her parents divorced when she was 4, she could never really handle it because it was really hard for her, even though she took visits back and forth cause "at the end of the day when she got home it just wasn't the same." And she said she "couldn't really understand why they divorced. She stated that there was an occasional couples quarrel but nothing serious, so she thought they were happy. So apparently it took a big toll on her to the point that growing up in her childhood life it became really hard for her to connect with anyone and come out her shell so she was really unhappy. She told me that she use to get into a lot of arguments with her mom when she was 12 and it was "crazy" how they would argue so often, which got her even more depressed. Which she then states messed up her grades in school, "everything was just so out of control cause she had a really difficult childhood life."
That's such bull. When I was growing up my parents didn't get a long either, my parents use to argue a lot with each other as well, they use to fight. Fist fight, I would be able to hear them from the next room. They divorced and dad wasn't around but I didn't get down from it. If I wanted to I could just get a train and go see him. I did pot and hung out but it didn't affect my grades. I did more bad stuff but I didn't react the way she did. So How come, me and this girl basically been through the same but it didn't take a toll on me.
Why do some people over react to stuff other people wouldn't really care about?
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