There are a plethora of reasons to either do or not do practically anything reasonable. Of course, what people define as being reasonable can vary widely from person to person.

Due to the broad nature of the word, the relevant definition for drugs in this is any governmentally controlled substance for which the use is either restricted or banned (ie cocaine, LSD, XTC, marijuana).

The first, and in my opinion, best reason to not do drugs is that you choose not to. Either you just have no interest in even trying them, you've tried them and don't feel like doing them again, or the idea of trying them makes you uncomfortable. Whatever your reason for choosing not to, stick by it. In any situation where it's purely a matter of personal choice, and about something that will affect only you, then keep to your choice. But if letting others push you into things is more appealing to you, then that's your choice also.

The second, and more health-conscious, reason would be quality control. In the world of controlled substances, there is almost no such thing as quality control. From '70's tales of people buying oregano thinking it was marijuana, to LSD having traces of strychnine, to the possibility of heroin being impure, there are myriad things that can be bad with any black market product. In many cases, a lot of people handle the drugs between the time it's harvested or produced to the time it reaches the end user, and the majority of end users are not equipped or do not have the knowledge necessary to test what they're using.

Another reason to abstain is that it's against the law. Regardless of whether you agree with the law or think it's wrong, if you get caught, you will be punished. Punishment can range from a minor ticket and record to siezure of property and years in prison. The punishments vary according to the substance involved, how many times a person has been caught, and how much substance is involved. Another thing to consider is that in some cases, other people are sometimes punished for drug crimes of which they have no knowledge or control merely because they were present at the time of the actual user being caught.

There are quite a few other reasons why one might choose to not do drugs. These might include religion, respect for others' wishes (such as parents or significant others), or lack of knowledge of their full effects or interactions. Some might be considered better than others, but in the realm of personal choice, any is as valid as any other.