When you set a target allocation of, say, 60% stocks and 40% bonds, you decided that mix matched your time horizon and risk tolerance. After ten years of unequal returns, the portfolio is no longer that mix. If stocks compounded faster than bonds — which they usually do in long bull runs — the portfolio quietly drifted to 75% stocks and 25% bonds. You did not authorise that. You did not pick it. It happened because the math of compounding inside an unattended portfolio always pulls weights toward whatever has run hottest. Rebalancing is the act of refusing that default — periodically pulling the weights back to the shape you actually chose. The cost of not rebalancing is not paid in fees. It is paid in concentration risk you did not consent to.

“Drift is what your portfolio becomes when you stop choosing what it should be.”