Why do we choose to focus more on negative?

A friend of mine was out on a date , which she said was going great, until her boy-friend brought up an old incident and made fun of how she had reacted then. “It left me teary eyed ….. I felt humiliated and it spoilt my evening completely,” she said.
The boyfriend apologised profusely, assuring her that he meant it as a joke and did not mean to hurt her. The laughter and the good times that had preceded that incident almost ceased to exist for my friend and all she could think of  was the jibe that had been made. In return she showered him with angry comments.

The incident left me thinking, in life, do we prefer to dwell more on the negative than on the positive? A great relationship, job or a friendship suddenly seem less than they actually are, the moment there is an unsavoury turn of events. Many of us find it impossible to forget that incident or forgive the people attached to it. If we do not reward our friends, partners, colleagues constantly for the good things they do for us, why punish them the moment something goes wrong? To step back and look at the bigger picture may show truly insignificant that one negative incident is, in the face of all the good times.