Detached object and attaching again. Hibernate

I am writing that post as a reference to:

JPA/Hibernate: detached entity passed to persist

The author has problem with exception:

Caused by: org.hibernate.PersistentObjectException: detached entity passed to persist: com.paulsanwald.Account
    at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(

And, the solution is:

As per the articles in the previous 2 links you need to fix your setters in both sides of the bidirectional relationship. An example setter for the One side is in this link.

My questions are:

  1. Why object was detached?
  2. Ok, object was detached. But, how repairing consistency solve that problem? The object must be still detached.
  3. Ok, object was detached. Why Hibernate does not attach it to the new session?