The instinctive reaction of pretty much all "nice guys" is to become defensive when she shit-tests you.

In essence, you react to her shit by winding yourself up. Before you know it, you're having a screaming row.

While we have a number of suggested ways to deal with shit tests (which I won't go into here; there's plenty of good material on the sidebar), I think it's important to consider why we're coming at it from this angle - particularly for newbies who may not have fully grasped this yet.

You cannot control every word your wife says to you. Nor can you stop her from shit testing you; half the time she doesn't even know herself that she's doing it! There's no point in getting worked up over something you can't control.

What you can control is how you react to it.

This is how you become her rock.