If you are playing blackjack against a single player, there is no reason to double. In fact, doubling can often be counter-productive because it can increase your chances of losing.
However, if you are playing blackjack against a dealer, doubling can be a sound strategy. The reason is that the dealer will almost always give you more money when you double than when you stand.
In fact, in most cases the dealer will give you at least twice the original bet. This means that doubling is generally a good strategy when playing blackjack against a dealer.
