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I checked out this novel, probably young adult, from my hometown library sometime in the late 80s or early 90s.
The plot follows a human looking race of aliens that come to earth to investigate why our race "only" lives 70-80 years (the aliens have a lifespan of several centuries, and have to use personal journals to remember the details of their lives).
Someone decides the moon is involved in determining our lifespan since no other humanoid race lives on a planet with such a massive satellite so close. The climax involves one of the aliens going crazy and deciding to destroy the moon to "save the humans". Another member of the research team thinks destroying the moon will horribly damage the earth and also envies humans for savoring our short lives so s/he attempts to stop the first.