Optimism vs pessimism

I've been pondering this lately. I am a pretty optimistic person. The glass is half full, things will work out, bad times don't last, etc.
I know people who are the exact opposite. Glass is half empty, things will always go wrong. You know the type. Maybe you are the type.

So, what I'm pondering is what makes some people inherently optimistic and some pessimistic? Is it nature? Nurture? Upbringing or genetics?

Confusing my thoughts on that issue is the fact that my brother is on the pessimist side of things. Same genetics. Same upbringing. What made us so different?

My husband and his sister are also opposites, with him being the more optimistic one. Now, he's not a perfect optimist. He still will assume that things will go wrong, places of business will not be open on holidays without checking, things like that. I'm not sure his sister ever sees anything good in life. Again, same parents, same upbringing.

What's the difference? What makes on person from a family be happy and optimistic and the other be pessimistic and unhappy?

I personally can't remember ever being a pessimistic person, except when I was a teenager. And any teenager worth her salt is angst ridden, downtrodden and pessimistic. Even then, I was still mostly cheerful. I just had my moments. So clearly I was either born with this personality trait or learned it very early on.

I've had the thought that even pessimistic people can learn to be more upbeat, but I don't know that for sure.

So, my deep thoughts for this week, what makes some people pessimistic and others optimistic?