There are many reasons why people try drugs. Teenagers with ADHD often try them if their condition has gone unrecognised or if prescription drugs have been unhelpful. It has been known for years that street drugs often help them feel better, but as their bodies become tolerant to the drugs they become addicted like anyone else. Adults often try them because of divorce, or money problems, or just because life is sh**ty and they think it will help them feel better. Musicians and artists are tempted to try them because it makes them feel more creative. They're all equally at risk to slide into addiction. 

It's easy to be dismissive and judgemental if you never knew these people before addiction and have never known an addict. But the fact is these people are sick. And the mark of a civilised society is that it looks after the worst-off in society and doesn't just discard them. And some - even the worst - are salvageable. So I support safe injection sites.