> some of us shift between the two.

I think ALL of us shift between the two. or rather I'd say that we all experience a combination of both over our lives

when people shift between opposite states way too fast, from a big and deep depression to a high excited manic state that's called bipolar disorder. but normal people can and indeed are (is what I'm saying) expected to experience both extreme but possibly shifiting between extremes over the course of many weeks or even months, rather than a few minutes like in bipolar patients