Why do we stress?

I woke this morning, at 5:15am, and throughout the day have simply not been able to shake the feeling that everything sucks and is all going wrong. It is a very unobjective and rather incorrect view but I just cant get rid of it. Seriously. I mean we are on top of things, kind of. The things we aren't on top of aren't life or death, nor close. Why would one, that'd be me, stress so much. Really, and it doesnt take an ingenious interpretation, I have had and continue to have shit fall in my lap. Jobs, a loving wife, travel, opportunities, new skies etc....and it is a long list. Yet, a little form, the possibility of something going wrong and I freak out as if I had cancer....i mean what is with that. It's not even real, it's just the chance that it could go wrong...oh my god!! anyway...I've said my bit...so why??