It's not the experience, it's how you remember it.
Apparently we are creatures that are affected more by our memories than the actual events. Our memories are focused on the peak of any given experience, and the end of that experience.
The examples are clinical, from the book I've been reading.
Volunteers put their hands in painfully cold water for 60 seconds, and 90 seconds. The first 60 seconds of the 90 second version were the same, temperature-wise, as the 60 second version. The last 30 seconds were one degree warmer, and so ended with a less painful state. When the volunteers were asked which experience they preferred to repeat for the third trial, they said the 90 second version.
Similar results were found for painful surgical procedures - what mattered was the average between the peak of the experience, and the level when the experience ended.
It's not logical, but we aren't logical. We have a pretty good, but flawed, way of making decisions. We have a pretty good, but flawed, way to record a lifetime of more experiences than we can actually record in detail.
So... the most successful vacation has enough time to allow one to relax, a strong high point, and a good ending. And the photos to bring the memory back up again.
The most desirable lifetime is one that has no major stress, some high points, and a good ending.
Which makes one's desire to plan for one's golden years quite understandable. It's not who we are dating right now, it's who we'll be with when our life is over, and what the overall narrative of our life was.
What path will give us the memories that prove we had a good life, and few regrets?
And does this mean that it's better to have many short term relationships as long as they end well?
I don't know... I'm still trying to figure out how to use this knowledge to overcome the flaws in my decisions, find happiness, and give my story a Hollywood ending.