I think happiness can only exist in conjunction with unhappiness as a comparative value: If you don't know sadness, you probably won't even recognize if and when you are happy. So I think absolute happiness is not desirable.
I think this iidea is a way of coping with life being unfair. Someone, somewhere has had a relatively dream run of life and been very happy a lot of the time. Lots of friends, met the love of their life, achieved their dreams. Saying this person couldn’t have truely been as happy compared to someone who lived through constant tragedy, hardships and only got to experience a small glimmer of happiness seems far fetched.
I just think that some setbacks and obstacles in achieving one's dreams, which make one unhappy from time to time, are necessary in order to properly appreciate what one has achieved, which in turn gives rise to a feeling of happiness. I'm not so much concerned with a gauge of happiness that could measure how happy someone is in relation to someone else, but just that you also need to know what the opposite of happiness feels like in order to realize that you are happy. If one were only happy all the time, it would probably be a boring state of mind.