The Pain of Finding Your Ex in a New Relationship

When you find out your ex is in a new relationship, not everyone feels that way. Some people subconsciously are comparing their journey of healing to their ex's journey of healing. Not everyone has the same reason, but there are many reasons behind it. I think for some people, it means that it's absolutely final. For other people, they draw those comparisons and they get a lot of pain thinking like how is this the dynamic?