Rebalancing forces a systematic 'sell high, buy low' discipline that deliberately fights your own instincts. Left alone, a portfolio's actual mix drifts over time — if stocks have a great year, they'll naturally grow to be a larger share of the portfolio than originally intended. Rebalancing means periodically trimming what's grown and adding to what's lagged, to return to the original target allocation.
It often feels emotionally uncomfortable — selling a winner and buying a laggard runs against most people's instinct to 'let winners ride.'
Correct moves you up, wrong moves you down — reach 100 to master this lesson.