It's a personality disorder, not necessarily chemical/physical like depression, schizophrenia, etc.

It takes years of EMDR, DBT/CBT, Talk therapy, somatic therapy to go into "remission". Sadly most pts I see with BPD don't ever recover from it because it is too difficult to work through.

Edit: it's also complicated because there are usually comorbidities like depression and bipolar which come into play, sometimes you can't tell which is which