We know a personal connection is critical when leading others, but sharing too much can undermine authority and create uncertainty. After all, imagine your manager saying, "I'm scared, and I don't know what to do."
There are two times when it is valuable to open up. The first is when it will be helpful to others. For example, consider how the comment would feel to you if your boss made it. If you would be grateful to hear it, probably your team will, too. However, err on the side of caution. For instance, letting employees know you're in a bad mood because you're having a lousy day is fine; telling them you're in a bad mood because you disagree with a decision by senior management probably isn't.
The second time it is beneficial to open up is when it helps your team feel less isolated. For example, if you suspect people are apprehensive about a project, admit you're stressed about it, and thank them for their hard work.