I think there are good evolutionary reasons why this should be so. Observe a wild animal in nature and ask why it is 'wild'. It is in constant fear of predators. It must be in a continuous state of high alert as it scans its environment. Its life is a daily struggle for sustenance and survival. We evolved from those conditions and I think our minds are still attuned to this struggle. The struggle is the baseline against which we experience the contrasting emotions. The contrasting emotions are the reward that makes the struggle worthwhile.
When we take away the struggle we disable our capacity for joy. We try to recreate that joy by feasting on the modern cornucopia of plenty. When this predictably fails we become stranded in apathy, disillusion and depression, becoming dysfunctional.
Turning now to the high state of alertness of the wild animal. This is entirely missing from our lives. We have made our lives safe. I think that this state of alertness demands a focus of attention on the environment, a curiosity about the environment and less attention to ourselves. Remove this need for alertness/curiosity about the environment and our attention moves obsessively to ourselves with the same results I mentioned above.
In conclusion, I think a happy life is a resilient life with worthwhile goals, focussed by an intense curiosity about the environment. I use the word 'environment' in the very broad sense of meaning everything external to the individual. Resilient here means being regularly exposed to significant stresses and recovering from them(the suffering/joy seesaw).