Many at times people think opening up to their current about their previous relationship is a romantic move. One can feel like when they share such information, they are trying to create a strong bond.
However, this might not be always the case because it might be the source of another breakdown in your new relationship. When you tell one the flaws that you had in your past relationship, at times they will tend to judge you. For instance, if you were in an abusive relationship, one will be judging you in the fact that when you have a disagreement they tend to think you were the problem in your relationship.
Another reason why you should not share about such information is because you might get back with your ex anytime. When two people breakup, there is always a possibility of them getting back. Especially if they parted ways in a cool manner. So when you share the negativity of your ex then later on you guys reconcile, you will always be judged.
This will also make your bae to be comparing your relationship with your past. One can be like, 'you used to do this in your past relationship, so why is the case different in this??' With time you will find that you will be having so many conflicts in that they are sourced from your past.
Lastly,you can be sharing information about someone that is familiar with your current boyfriend. Guys get jealous knowing the fact that you were with someone whom they know. It can cause a great beef between the two parties. Either you and your ex or your ex and your current boyfriend.