It doesn’t decide to repress it … in fact the whole idea of ‘repressed’ memories is highly contentious.

It’s that it isn’t stored to begin with … or, at least, the access to the memory isn’t stored, the connection never made — the mind takes us to a ‘happy place’ in which the event(s) simply aren’t happening and/or, afterwards, didn’t take place.

It’s most likely connected to the fear response and pain — memory traces aren’t laid down by rats receiving electric shocks to their feet at the point in time when the learning stimulus is presented to them for instance.

And for those memories that are simply impossible to ignore, there’s active forgetting … refusing to think about it, thinking of other things, la la la la I can’t hear myself … weakening the strength of neural connections until we genuinely have trouble recalling more than the fact that the event happened.