I think there is an innate human bias to weight negative experiences more strongly than positive ones.

For example, I will forget a compliment a day later, but I will never forget that one time I was harshly criticized.

It makes sense from an evolutionary perspective as well, the worst possible experience (death) is far worse than the best possible experience (food,sex,comfort,etc).

Re: your example about politics. Politicans explot this by talking about how bad the others are, rather than how good they are.