There is no one answer to that question, because people are individuals and each is unique.
That said, there are common factors involved in addictive behaviours.
One common link is a desire to alter consciousness to a greater or lesser degree.
That can range from awkwardness in social situations needing a little alcohol to relax and 'take the edge off' right through to someone whose daily life is so unbearable, it needs blotting out for as long and as much as possible - and every variation in between
The problem with addiction is, every single addictive substance known to man, without exception, starts off being pleasurable - obviously, which is why people ingest more of whatever it is they like.
The problem occurs when pleasure turns into necessity, what was ingested to feel good is now ingested to stop feeling bad.
The addictive personality is the individual who is unable to recognize the tipping point, where one stops and the other starts.
But as I said, reasons and situations are individual, there is no simple pat answer to your question.