I hear this question quite frequently and would like to first explain Amazon's policy on this issue and then offer you some additional rationale and practical advice.
First of all, let me quote directly from Amazon Associates Operating Agreement:
The section above clearly states that you may not click on the link that contains your Amazon Associates ID and make a purchase from Amazon. However, it is not that simple. Several questions come immediately to mind. As you know from my previous post titled Does Amazon store cookies on people who bought through your affiliate program?, Amazon keeps the cookie for 24 hours after the customer clicks on an Amazon affiliate link. So, what happens if you have been clicking on your own links to test them then inadvertently went to Amazon later in the day and made a purchase? Obviously, you would be tagged with your own Associates ID.
Clearly, this is is not an intended violation of the agreement. What happens is Amazon reserves the right to withhold paying fees on products you purchase for your own use. If you clearly abuse this section and build a site via which you exclusively purchase all your Amazon products and offer your friends, relatives or associates to purchase Amazon products, Amazon reserves the right to terminate the agreement entirely (in other words, terminate your Associates account and not pay you any fees you've earned so far). So how does Amazon know when to withhold fees and when to terminate agreement? Because it is not easy to tell if your purchase is inadvertent or intentional I am only aware of Amazon withholding fees for personal purchases and not terminating the entire agreement for this reason.
The next question that comes to mind is why restrict such sales? Retailers that have an affiliate program generate only a portion of their sales via affiliate sales. Majority of all sales occur when customers come to a retailer directly and the retailer does not have to pay any affiliate fees on such purchases. Because retailers regard affiliate programs as a marketing channel, they hope affiliates introduce customers to their products and service, especially those customers who have not shopped with the retailer previously. Once a customer has a positive shopping experience, retailers hope the customer will return directly to them, bypassing affiliates and thus saving the retailer from incurring affiliate fees on follow-on sales. When the affiliate purchases products directly from the retailer with his own affiliate ID, the goal of the affiliate program is broken. That is why Amazon and many other merchants do not pay fees on purchases made by affiliates for personal use.
Then why restrict sales by affiliates to their friends and family? This a poor decision in my opinion. Obviously, if I encourage all of my friends and family to shop at Amazon, Amazon will get sales that they otherwise may not have received and will benefit from such sales. I believe that this rule was put in place because Amazon argued that as an Amazon affiliate, I would tell all of my friends and family to shop at Amazon even if I don't receive affiliate fees. I do not agree with such an argument. A merchant should give all of their affiliates 100% incentive to drive any and all sales to the merchant, including sales to friends and family. Thus, affiliates should be compensated for such sales. Furthermore, while I do see the merchant's perspective for restricting personal purchases, because of the difficulty to enforce such activity (affiliates could always open two accounts, one for personal sales and one for referred sales), I also believe that all such purchases should be allowed. Unfortunately, today Amazon does not permit such sales.