While there is certainly some biochemical connection between the thought processes (negative/positive), I think that some people are just raised to be more cheerful and optimistic. There are whole families who always see the bright side and other families who couldn't see the light for the angle of their heads.
Regardless, we do have the ability to alter our train of thought...if only we can remember to do it when we're feeling low. I've often stopped an oncoming bout of depression by changing my thought pattern.
What was very interesting was an incident where my father was really having a low. It was not long after my mother's death, and he had spent about three days on the couch in a deep funk. Finally, I went in to him and told him that his pain was overwhelming for all of us and invited him to give it to me. Within seconds he jumped up, changed into his visiting clothes, and went out to do his favorite thing: gather the gossip from around the countryside. His mood had made a 180-degree alteration in seconds.
I was desolate and cried my eyes out for hours.
This incident proved to me that we have much more control of how we feel than we credit. We just have to remember that how we feel is in our power. Many times when we are feeling negative, we choose to be cynical as well and not believe that we can impact our own feelings. At the risk of sounding like a Goody Two-Shoes---which I definitely am not---we are the boss of us.
