Here's my humble explanation:
I'm a neuropsych researcher, and with some simple deducing (and confirmation from Von Hase), I've concluded that INFJs have more heightened electrical activity in the limbic system of the brain. The limbic system is mainly composed of the cingulate gyrus, the hippocampus, and the amygdala. Together, these areas mainly contribute to learning, memory, and emotion. My area of focus here though, is the amygdala.
The amygdala mainly controls emotions, fear responses/memories, and "gut feelings". People with heightened amygdalar activity are known to have social phobias (sound familiar?) Now, when something really bad happens to you, because the amygdala is so small, it will take kind of a "flash-bulb" memory. This is a small and undetailed, yet VIVID memory and they are meant for our survival. That's why if you see a garden hose and jump because you think it's a snake at first, that's your amygdala initiating a fear response with a flash-bulb memory of a snake that you may have seen in the past. These memories are the hardest to get rid of because they are there to warn us not to run into the same problem. If anything though, do NOT keep retreaving these memories if you can help it. Every time you do, you're releasing cortisol into your bloodstream, a stress hormone that will make you sick over time. A typical situation for INFJs. Sometimes, there just nothing like a little bit of healthy repression. At LEAST until the intense crappy feeling goes away.
The hippocampus, on the other hand, is involved in emotion as well. But we are starting to learn that this is where our long term memories are consolidated after they're been rehearsed enough times in our prefrontal cortex. Though the connections may get muddied over time the less we access these memories, these memories will always be stored.
I think that we may have more electrically active limbic systems in part due to the fact that it's just where our dopanine and serotonin receptors seem to cross. But I'l have to look into that a little more.
EDIT: I'm almost kinda considering making this it's own thread. I'm suddenly curious how many people can relate to these heightened fear memories and long-term memories... ::shrugs::