Uninformed people sometimes casually make statements about moodiness or indecisi

Uninformed people sometimes casually make statements about moodiness or indecisiveness by saying things like “that’s so bipolar” or “you’re so bipolar.” After learning about Bipolar Disorder I & II, what do you think about the casual use of the word “bipolar”? Why might someone who is diagnosed with a bipolar disorder take offense or be upset by that?