after your divorce and before the baby's arrival, you stated that he was still being an obnoxious PIA and fighting you at every turn. and altho it was negative attention, it was still attention. and also while awful, it became the new normal.
now he has many other distractions. his focus is off you and onto other things. there isn't as much drama, conflict, or negative attention. he is not creating such adrenaline-inducing situations. you are no longer remanded to Fight or Flight.
the drop of adrenaline, the drop of constant conflict, those alone can cause a marked change in mood and outlook. it's a natural reaction to change.