It has to do with insecurity and jealousy. I don't think knowledge of before is a bad thing. Everyone has a history.
There are no guarantees when it comes to relationships (or anything for that matter)Everything has an expiration date. People come into our lives for different reasons and are meant to be there for certain amounts of time. Some relationships last until the death of one person (or both), some relationships last for years, some last for months, weeks & days. That's life.
The only thing you can do is focus on the present and work hard so that you have a future together. In order to do that, you have to let go of the past.