I used to be funny and confident but now I'm shy and anti-social. This happened to me due to a chemical imbalance in my brain. It can simply be treated with medication and is often the cause of change in behavior in many people. Also, If somebody had to undergo a lot of stress or perhaps suffers from post traumatic stress disorder, they may never be the same again. If you develop any mental disorders or needed to adjust to some type of dramatic change in life, you may become less outgoing and anti-social.