Learning more about people's pasts helps you understand what motivates them. Hopefully they're mostly full of fond memories. But inevitably, there are also uncomfortable ones that they'd like to avoid repeating, which are often the basis for their insecurities.
Insecurities make ordinarily level-headed people behave irrationally. Don't underestimate them. For example, when people feel jealous, they're prone to act out of character and do things they know are socially unacceptable like stalking and worse. Or, when they're feeling neglected, they'll act out to get attention.
Think about your own insecurities and how you'd want people to act if they knew about them. Try your best to put people at ease. Since insecurities are uncomfortable to talk about, you'll likely stumble into them by accident. Be patient and let others decide when and how much they want to share with you.