From what you've posted, and the suggestion that this problem didn't exist in your premarital relationship, my best bet is this...


Now that you two are married AND have a baby, he feels completely responsible for you and your happiness.


Everytime you express unhappiness, it is a slight to his ego, because it means he is failing as a husband, father and man.


So he can't bare to listen to any negativity, his defenses immediately goes up because your unhappiness is a direct reflection on him.


Maybe you could try a calm conversation about this angle and see if it resonates with him at all.


At worst, it rules out something, at best, you have a starting point.