“We don’t rebalance because we are first and foremost human beings with a multitude of considerations before we are investors and fiduciaries optimizing for long-term expected returns……Despite all of the intellect and adaptive learning that we bring to bear, sadly, human beings with our changing risk aversion are poorly suited as stewards for managing long-term returns.”
http://www.indexuniverse.com/sections/research/12647-why-we-dont-rebalance.html