A single stock made you wealthy, so it is tempting to believe it will keep doing so. The uncomfortable truth of concentration is that the position that built your net worth and the position that protects it are rarely the same one.
A diversified portfolio and a concentrated one can share the same expected return and have completely different ranges of outcome. Concentration does not necessarily lower your average result; it widens the distribution, fattening both the best case and the one where a single company's bad decade takes your financial plan with it. The question is not whether the stock is good. It is whether your life should depend on it being good.
Advisors sometimes quote a rule of thumb, no more than ten or twenty percent of net worth in one position, but a single number cannot answer a question that depends on your whole situation. What matters is how the concentrated position sits against everything else: your other assets, your income, your time horizon, your tax cost of trimming, and how much of your future you can afford to leave riding on one outcome. The right level for a founder with a decade of runway is not the right level for someone five years from relying on the money.
A useful review works through the real inputs rather than a rule of thumb: the embedded gain and the tax cost of reducing it, whether the holding sits in a taxable or tax-advantaged account, techniques that can manage the transition over time, and, underneath all of it, the honest question of how much volatility your plan can absorb. This is general information, not a recommendation about any security. The goal is a decision made deliberately, with your tax advisor at the table, rather than by default because selling felt like disloyalty.
The stock that built your wealth is not always the one that keeps it.