Lately I've been hearing this a lot, and it's actually making quite a bit of sense, experientially, though I can't really explain why.

I'm not only talking about how tremendous sorrow opens one up to tremendous joy because of it's ability to cause appreciation, or joy opening one up to sorrow because of its ability to cause loss. I'm talking about the two feelings actually being the same feeling. Does anyone have any experiences of this? Or possible rationalizations of it? I can imagine it seems plainly false to some, and I can't exactly discredit that opinion easily.

But does anyone believe the two can be experienced as the same? Or do you believe it can only work like a pendulum or a scale?