I don't agree that your conversation is representative of all ISTJs. That is a cardboard cutout of any human being. I'm surprised that's all you have, being an INF and all. I'm too tired for a paperdoll fight right now, but your post is exactly the problem with MBTI. It is an amazingly shallow comprehension of the human psyche.
Edit: Actually, if someone tells you they are sad "because of chemicals in the brain?", then chances are they are blowing you off. That could just as easily mean, "I am a private person, you are being intrusive, I don't trust you with my inner world, so I'm going to say the exact thing to annoy you most."