The "you can tell me anything" phrase can be dangerous in so many ways. It's often a tactic to break down the boundaries of whoever it's being told to, and it's not okay if the person you're talking to doesn't have any social boundaries of their own. If a parent says it to their child or a therapist says it to their client I'd expect that to be fine, but beware of the people who use it in a predatory way.
It's not just about being careful who you tell personal things to, it's also about recognizing when they're trying to get something out of you.
