Addiction, in medical terms is a condition in which a human body depends on something for its normal functioning, which further leads to physical dependence. Addiction triggers as a mental issue and proceeds to a physical reliance. In such a state a person tend to rely on a typical substance or a drug that seems to give him relief. An escalated drug tolerance is what we know as addiction. One can even get addicted to prescribed drugs; It need not necessarily be due to drug abuse.
It is quite tough a job to answer whether all drugs are addictive or not, it depends on how one defines addiction. Some feel that psychological dependence on a drug or any other substance is addiction. It can be anything. Some people are inclined toward these things and feel that they can not live without the influence of the substance. This is more of a problem with the person who is using it than the substance or drug itself. Dependence is after all a human nature; We can not blame a drug just because we depend on it. This has more to do with the mental state of one's mind than the physical.
Now, talking of physical dependence, it happens with certain drugs. We get used to some particular drugs typically and can not survive without it. This happens due to repetitive use of the same drug for a reasonable period of time and in heavy doses. Tranquilizers, heroine and also the common alcohol are the major addictives. These drugs are the most potent ones and are usually consumed in heavy doses and on a regular basis. Irregularity can lead to severe complications like flu, the shakes and at times can also lead to death. The body and he mind are totally dependent ion the drug and is prevention against illness.
The cause of drug addiction largely depends on the drug in use or abuse, the person using it and also the situations in which the drug is taken. It is the weakness of a person that leads to addiction. Some social issues are also being held responsible for drug addiction. Mental weakness, peer pressure, emotional set back and inferiority complex, any of these can be responsible for addiction. Drugs can not be blamed.
It is true that tranquilizers have an addictive effect, so are some of the commonly used cough syrups, but that happens incase of an overdose. Normal dosage is fine and helpful too.
One should not get messed up with the terms drug-dependence and drug-abuse. These are totally different terms and have different meanings. Dependence is the after-effect of stopping the use of a drug, while abuse refers to knowingly over-using a drug. The former is more tolerance and the later is a chronic symptom.
Seeping pills and other drug like ecstasy, LSD and cannabis, when used regularly can cause psychological dependence, physical dependence is unilaterally. Drugs, which are more of stimulants like crack, cocaine, nicotine and amphetamine also lead to psychological dependence, the physical dependence of these drugs, are yet to be proved.