At this point, I'm left wondering why humans evolved to be so gosh-darn negative all the time. It feels like there must be some hidden upside to being negative that just hasn't occurred to me.

It's not so much that there's an upside to negativity as that continued positivity is evolutionarily useless. Evolution wants you to "chase the dragon" of steep, exciting highs rather than maintain a reasonably happy steady-state or, worse yet from Its perspective, "go full transhuman" and rewrite your own mind-design to bring Being Happy and Doing the Right Things into perfect alignment (which we can't do yet, but probably will be able to someday).