Addiction
Most people believe that someone becomes an addict because they progress from using drugs recreationally into a regular habit, driven by an addictive quality embedded within the chemical profile of their substance of choice. Let's address this right now, in one word.. Horseshit!
A perfect example: A huge amount of American soldiers participating in the Vietnam war were using heroin while stationed overseas due to it's ease of accessibility, and it's provision of an escape from the harsh reality they were living in. Over 95% of them never touched it again after returning to their lives back home once their service was over.
The reason for an addiction exists within, long before the substance of choice makes an appearance.
Addiction explained
An addiction develops only as a coping mechanism of some sort, where there is some underlying pain inflicted by some trauma earlier on in your life, predominantly from some time in early childhood. It doesn't always mean it was some tragic horrific event like abuse, as even something you would consider negligible as an adult, can be far more impactful when you are young, the result of which could embed itself and reside within, solidifying in nature the older we get. Should this have created fear or pain which makes us uncomfortable to think about and deal with, our mind starts silently building a coping mechanism to handle it for us, and eventually tucks it away out of sight where it can't hurt us anymore. However, the mind will eternally need to allocate some of its resources to to keep it there, under wraps, so when you suddenly introduce a chemical which overrides the effort needed, your mind, whose modus operandi is to operate as efficiently as possible, spots that it can recoup and free up this mental effort, by allowing the chemical to take over the pain management function. instead of it.
And so begins the addiction.
The powers that be all agree that at this point, you are fundamentally no longer taking drugs to get high, but rather you're taking them to get numb.
From here on though, while the overall rule mentioned above remains, the nuances, and various micro-options that exist in around this subject are so varied and vast that one should make their own mind up about it by reviewing the data that's out there and painting their own picture.
Watch the videos on the right by the world renowned Gabor Mate and Carl Jung, which are an excellent place to start and portray very compelling theories around the structure of addiction.