well, it's easier to look back at things, and only remember the major events. Often times life is boring and unhappy from the day to day pattern, but major events come and interupt it (like a vacation or a family tradegy) so if most of the major events of a time period were positive you will remember it as a happy time, even though you didn't much like the day to day life you lived at that time.
I think the real key to personal happiness isn't accomplishing major life goals (write a novel, travel the world, etc) but is instead, finding a day to day life pattern that you can enjoy.
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