There are many people who like to be very secretive. This is a very unsettling trait for the other indeed. When you are seeing someone, you are a couple, you have to be open with each other. Yes privacy is important as well but there has to be a balance as well. If someone is wishy-washy about his friends, or job, family, or other aspects where he or she should not be, then that is a red flag you need to have a conversation about.