Answer:
Once the heroin gets into your blood stream it goes straight to your brain and takes over. The drug takes over the physical dependence and blocks the sensation of being rewarded for the body. Thus, a steady user of the drug cannot feel any good feelings without the drug in their system. So people that use the drug get addicted because without it they cannot feel happy feelings.
More specifically, heroin (and most other addictive drugs) affects the dopamine system of the brain which is responsible for feelings of pleasure and reward. Initially use will increase the amount of available dopamine resulting in feelings of pleasure. Continued use will "down-regulate" the dopamine system making it less sensitive. When drug use is stopped, dopamine levels fall back to normal. Because the system has now been desensitized, the user will not feel the same degree of pleasure from activities such as sex, eating, etc. (anhedonia). It may take many months, however, the dopamine system will usually recover and the user will feel better/normal. This effect is partly responsible for psychological dependence on heroin.
Heroin acts on other systems of the brain and body which can result in physical dependence as well as the psychological effects noted above. When use is stopped, a very unpleasant "acute withdrawal syndrome" may result, lasting for days. This syndrome is probably what most people associate with heroin addiction, as "dope sickness" is often depicted on television and in moves.
Physical dependence makes it difficult for an addict to initially stop using the drug. Psychological dependence makes long term abstinence difficult.