Overreacting to possible threats is one example. Recoiling from a bitter taste or fleeing from a rustle in the bushes might have kept an ancestor from death by poison or tiger attack. Negative emotions alert you to danger so as to avoid immediate peril, and there's little harm done if you react to a false alarm, spitting out radicchio or running from a bunny. But what used to be good for survival doesn't translate well to the modern world, and over the long term, repeated or constant revving up of your fight-or-flight response can lead to anxiety, unhappiness, and health problems.
Another theory relates to sensitivity to rejection. Early humans lived in small communities in difficult conditions. Being excluded from the group could literally mean death. As a result, humans are naturally sensitive to being socially excluded. Today, however, frequently feeling slighted or jealous can have a negative impact on friendships, marriages, and other social relationships.
It helps to recognize why it takes some work to counter these hard-wired attributes, but just because they're "natural" doesn't mean you have to be ruled by them.