I think it's impossible for a person to be only one thing, so single minded, and your example is a good one. It's unlikely the human species would have endured had we been that way from the outset. A person being one dimensional is merely a convenient word; sure, they may have a penchant for seriousness as you say, but they'd not make it through life being that way all the time I think.
People are multi-faceted, it's a simple fact, although many defer one aspect of their personality a lot, like the negative side for instance, the aggressive or dark side.
An interesting topic I think.
Ditching the negative is more about finding balance. One isn't positive by default simply because they avoid being an asshole. They're just somewhere in the middle. I think most reside there, naturally, until something knocks them out of balance. A big smile is a sign you're off balance just as much as a frown. Everyone naturally settles back to the balance, if they're healthy of course.
Today for instance someone said I'm polite. I can think of times I've been rude. I wasn't trying to be polite either. Can't recall a time when I was trying to be rude. These things just come out naturally in the moment. Of course I'd prefer to be polite. And there are times people find being rude to be acceptable. Someone pushes you, you don't say, "Sorry." You say, "What the fuck man watch where you're going." Then they say sorry and we're back to balance again.
Yep, humans are weird and complicated, but also somewhat predictable I guess.