People can be described as having emotional rollercoasters, but it seems to me that people with bpd have emotional switchboards. Push a button and sadness turns to anger. push another one and all emotions cease. I never grow angry then wind down. I'm either angry, or I'm not. The other day I went from angry to calm in the middle of an argument. The funny thing is that I was still thinking logically through the whole thing, but couldn't stop being bitingly sarcastic, even after being told twice to stop.