He lived for the drugs and he hocked everything he owned for that next hit. And the drug had less effect each time he used it.
Drugs are pretty insidious. They replace the desire to live. Why? Because they replace the very chemicals in the brain that make us want to live. That motivate us and that give us pleasure and gratification. Some of the drugs are very targeted and specific. It is very easy to see why someone would get addicted.
If you wanted to destroy a person, this is the perfect tool.