Why do daughters of single teen moms repeat their mom's mistake by becoming teen moms themselves?

Scrolling through my Facebook, I see my friends sharing posts from teens (17-19 years old) announcing that that they're pregnant. I've seen several girls who have broken up with their boyfriends, and now they have to take care of a new life, while fresh out of high school. These girls are also daughters of teen moms themselves. If you grew up seeing your mother, who had you when she was 17, struggling, why would you go on and repeat her mistake by sleeping with a guy who doesn't care about you?
Why do daughters of single teen moms repeat their mom's mistake by becoming teen moms themselves?
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