Waiting and thinking

My boyfriend is still waiting for his phone call and when he is leaving for basic. I have been thinking a lot about it. Why is it the things we dread or worry about are the things that we worry about the most. I really want him to go because I know it will be a good choice for him. At the same time I am worried of losing him. That something will come between us. I wonder why we don't think about things that make us happy or upbeat in some way. We always think more about the bad things that could happen, or the things that we are upset about. Our emotions have a huge effect on us. But why do our brains or just us thrive on the bad things.