Have you ever thought of what happens behind closed doors? Have you asked yourself what happens to a door that has been shut against you or a door you shut against someone? The word door is ment in a metaphor.
For Example when you get out of a relationship, do you ask yourself how the other partner is doing? Are you curious about how he or she is living their lives without you and what they have been up to?
There is a saying that; curiosity kills the cat! There is a reason why you left the relationship and that reason was strong enough for you to breakup. A closed-door should remain closed, most of the time, there is nothing new there for you to find. Looking back will get you no where but depriving you from moving forward.
When a door is closed for a serious reason, then let it be closed for good!