Boredom is commonly mentioned. I was gonna say, "To keep occupied", so that's boredom I guess. I have always gotten into whatever I have gotten into, whether it be Ham Radio, computers in the day, the occult when I was 12, whatever, with a passion. A friend once said that I don't have hobbies, I have obsessions.
Drugs are a complex and multifaceted subject, also addicting on their own. So, there it is.
As with so many peeps, pot ended up being a manageable addiction which didn't ruin my life. So, it stuck. There's also a lot to learn and obsess about with pot as well. Perfect.