I believe that living a happy life is a long term survival strategy. Producing offsprings is the other way. If we feel miserable, we want to cut our life short or it will be due to many problems, bad health included. Long term happiness is a very good measure of how good your life is going. So while we might not agree on whether we should optimize our lives for happiness, it's certainly an objective we should always consider. However, we should not confuse happiness with pleasure. One is meant for the long term, while the other is ephemeral.