There are two schools of thought when it comes to dividend reinvestment. One of the options is to automatically reinvest dividends, whereas the other option is to selectively reinvest dividends received.
The automatic dividend reinvestment is the easiest one to do. Once you purchase a dividend paying stock, you essentially check the "dividend reinvestment" box. As a result, your dividend income is reinvested into more shares of the same stock, and you start the income compounding process. The set it and forget it type of action is particularly appealing to income investors who are just starting out and have small amounts to invest in the beginning,
Because of the fact that it is free, automatic reinvestment into more stock is most efficient for those investors. Otherwise, even at $4-$5/trade, reinvesting anything less than $800-$1000 in dividend income would be prohibitively expensive. In addition, some companies offer DRIP discounts for shareholders who automatically reinvest distributions back into more stock. Unfortunately, even if you reinvest dividends back into the same stock that paid them in a taxable brokerage account, you still owe taxes to the IRS, depending on your income level.