Because addiction is so very powerful. It strikes our chemical makeup and affects our pleasure receptors to the point that we constantly seek that feeling. If we aren't strong enough to realize and accept the "feel good" we get that is built-in to our environment, i.e. family and friends, and accept that reality is imperfect and we will not always "feel good", it is much easier to turn to drugs and not care about the things in our lives that are truly important. Addiction cycles are very difficult to break. It takes a very strong willed individual who is willing to accept that they have made a mistake, and almost always, they will have to be able to reach out to others for help.