Person W lives for 50 years and is almost blissfully happy. He or she has an average happiness of 9/10 across their entire life.
Person X lives for 80 years. For the first 50 years, he or she has an average happiness of 9/10. Then they live for another 30 years at an average of 8.5/10 (not so close to bliss, but still very happy). Thus, person X gets everything person W gets and more ... but is less happy on average over their total life.
Person Y lives for 35 years at 8.4/10 (yay, that's very happy!) and then 50 years at 9/10.
Person Z lives for
All right, got it? Whose life would you prefer to have? W? X? Y? Z? It is by no means obvious to me that the best life to choose is that of person W, even though this person's life is the one of greatest average happiness across an entire lifetime. Indeed, it seems obvious to me that it's better to be person X, but maybe you'll all disagree with me. But what about person Y, and if you like that, why not person Z? And of course, we could come up with a more systematic set of comparisons if we were being scientific.
Are these even the relevant comparisons we should be making? Is it relevant to your choice if I tell you that one of the above lives (but still with the average figures I've provided) includes times of horrible pain or mental suffering? If it's relevant, what effect does it have on your choice?