Insecurity

After finding out who your ex is, your new boyfriend is jealous and insecure. Why is that? Is it not clear that you'd rather be with them rather than with your ex? I don't understand how you can have a full conversation about how much they mean to you and how much you don't want to be with your ex yet they still withdrawing from you. I'm insecure about my weight but I wouldn't be insecure about my man's ex because its clear to me that it ended for a reason. Now, unless they were chilling and hanging out alot, maybe i'd be a little insecure but if not, why stress yourself?