One of the ways I like to look at it is, humans have the unique ability to look into the past, and into the future. With this ability and the help of language, we are able to replay past experiences (good and bad) and make predictions on the future (plan forward, avoid things that hurt us before etc). This can cause “stress” or “pain” on the brain/person when those thoughts are negative. As things normally do, this “stress” can vary from person to person, depending on their makeup, life experiences, habits etc.