People don't always tell you when something's up, so you need to be vigilante around people when you notice something's different. Sometimes that means being a shoulder to cry on, sometimes that means keeping a secret. It's also important to know when it's okay to keep a secret and when it's not. My friend (this was ~2 years ago) told me at school she was cutting and she told me to not tell anyone. I recognized that this was the kind of secret that can be dangerous if kept completely secret, so I told my mom and she called my friend's mom. My friend thanked me the next day because it made it so much easier for her to talk to her mom about it. Now she's in a better mental state and is way happier than before. It's important to know that when a secret can cause harm by being kept a secret, you need to tell someone who can do something about it. This doesn't mean tell everyone, it just means tell someone who can affect the outcome and potentially save someone's life.