system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a brain pick out what memories are traumatic enough to block out?
gpt
When a developing brain is flooded with enough of the neurotransmitters (the chemicals we use to think and feel) most strongly associated with negative emotions like fear, sadness, and pain, it triggers an instinctive reaction to cut off the connections with the memories formed in that section of brain cells. A brain has a lot of trouble finding memories that are cut off this way, like a blockaded section of a city.

This is a survival instinct selected by evolution, which means that kids who didn't tend to do it died off from not being able to deal with all that trauma. Those who happened to respond this way survived and grew up to have their own kids, passing on a tendency to react this way. That's also why adults don't do it as readily: we've developed better ways to cope with trauma over the decades of our lives, so we're less likely to die from it (also that adults are more likely to have already bred).