I recently came across an article regarding recent research, done in the UK, which showed that in many people (80% of those tested), the brain preferentially processes positive thoughts over negative ones. Of course, that might seem obvious- there's really no mystery as to why we would prefer to remember or accept something positive over something negative. However, in some people, the positive beliefs are maintained even after learning the (negative) truth; in effect, the brain simply rejects the notion that the negative thoughts could be true, and sticks to the positive ones regardless of the reality.
While we'd like to think that we're capable of objectively and logically processing information in order to arrive at an accurate understanding of a situation, our brains themselves might be thwarting the efforts at objective analysis. Most of us have a natural bias in favor of positive news.
Our brain is constantly "testing ... predictions against reality" (Dr. Chris Chambers); in this case, specifically, subjects' brains tested empirical, statistical predictions against their perceived reality, but sided with whichever was more positive.
I would like to see additional evidence for this claim, however, as this study was conducted with only 14 subjects. I do not have doubts about the existence of this tendency, but I'm very curious as to what can be discovered about how or why this tendency developed. Other studies have showed that optimism is good for one's health, and can even contribute to longer lifetimes (http://news.bbc.co.uk/2/hi/health/8193180.stm), which makes me wonder about optimism's evolutionary benefits; or, possible disadvantages, as unrealistic optimism can hamper risk-assessment. I also think more in-depth research would be necessary to determine the variation of this tendency among different age groups, as instinctive responses to the idea of death, for example, varies greatly over the course of a person's lifetime. I doubt a group of 14 people was diverse enough to represent the various biases inherent in different ages, genders, or cultural groups.
Furthermore, research published in January of this year showed that there had been serious limitations with previous research on unrealistic optimism, and, though the University College of London study was conducted in June of this year, it is not unlikely that their research suffered the same constraints that earlier studies had.
News Article: http://www.bbc.co.uk/news/health-15214080
Abstract (full text- paid access only): http://www.nature.com/neuro/journal/vaop/ncurrent/full/nn.2949.html#/author-information
Abstract regarding uncertainty about optimism bias' existence: